Sri Lanka has so much to offer. Depending on the time of year, certain beaches are good for surfing at different parts of the island. It's also primarily Buddhist (except in the north) so there are a lot of beautiful Buddhist temples and rituals you can partake.
If you decide to stay longer than a month, you'll need a visa extension, which you can apply for almost right away after you've landed. You'll need to go to the immigration office to apply. It's an all day process and you have to go in person. Unfortunately there's no other way around it. I would suggest doing this when you first get in so you don't have to return to Colombo to do this later on on your journey. This guy does a pretty accurate job as describing the whole experience: https://www.whereintheworldisnico.com/assemblage/how-to-extend-a-sri-lanka-tourist-visa
A beautiful buddhist temple situated near the Gangaramaya Park. It's a peaceful and calm place to go and to get away from the city.
Mosquée Jami Ul Alfar
We stumbled across this beautiful white and red mosque when looking for Pettah. It's a pretty unique mosque and they were very inviting to let us take a glimpse.
A bunch of open bazaars and markets. Not necessarily for souvenir shopping, but this is where people go to get what they need for their daily lives. It's fun to walk around and check out how normal people live in Colombo.
I think this is the name of the neighborhood. I stumbled across this area on one of my last days in Colombo and remember it being quite cute and charming. There was a store called Barefoot that we went to and it had all sorts of prints and textiles. And the surrounding area was super nice! It's a little more modern vibes than Pettah.
Colombo City Hostel - If you need a day or two in Colombo to get acquainted, this is a great hostel. The rooms and bathrooms are pretty basic, but there are a lot of other travelers coming through and you can get some good recommendations. You'll begin to realize that there's definitely a backpackers loop around the country, but it's a great place to get the best current recommendations for accommodations in other places.
If you don't want to stay directly in Colombo, a lot of people stay in Negombo, but I choose to stay a couple days in Mount Lavinia south of the city. There's a good wreck dive pretty close to Mount Lavinia. I stayed at Colombo Lavinia Beach Hostel when I was here. It's run by a family in what feels like someone's home. It's less of a backpacky vibe if that's what you're looking for.
Kite Surfing - I spent a week in Kalpitya learning to kite surf. It's a beautiful and great place for this, but it definitely takes some time to learn. I stayed at a nice little place called Ruuk Village. However kite surfing is seasonal, so you may want to check with them if it's the right season.
Anuradhapura (a mouthful, yes) is a great place to bike around to see some of the well preserved ruins, temples, dagobas (Stupas), and small villages. Best way to get around is by bicycle. I think I only spent a couple days here, which may be enough to explore the area.
A short tuk tuk ride away, this place was magical during sunset. There are three points of interest at the top of three small hills with some amazing views of the area.
If you're feeling super adventurous, Jaffna is a pretty interesting place to visit. It's the most northern part of Sri Lanka and where a lot of the civil unrest occurred. This region is primarily Tamil and despite rumors of it being unsafe, I found that people were actually very friendly and happy to see visitors.
One of the main points of interest in the city. I thought it was absolutely fascinating. Though it mostly looks like an old fort which is in disrepair, its easy to picture what happened there with the rubble that now makes it look like ruins.
Delft and Kayts Island
I didn't get a chance to go here, but I really wanted to. I heard there are some old abandoned buildings left behind by the Dutch from when they occupied the region. However if you walk around some of the residential areas of Jaffna, you start to notice a lot of abandoned structures everywhere.
Accommodations - I stayed at a shithole hotel with some other travelers I met, so I don't have a recommendation for Jaffna.
I believe their season starts around End of March / Early April, but Trincomalee is a nice beach area to relax. I didn't stay in Trincomalee proper, but in an area just north called Nilaveli Beach, however there seemed to be a lot of cool places to check out in Trinco itself. There's a Hindu temple that overlooks some cliffs that looks particularly interesting.
From Nilaveli Beach, you can hire a boat (or a tour) to go to Pigeon Island, which has some excellent snorkeling! We spotted sharks, turtles, all sorts of beautiful coral. You can literally snorkel around the entire island and there's also a very short and nice little hike you can do.
Fernando's Bar (in Nilaveli)
A chill bar right on the beach and a popular hangout with other travelers. They also offer cabanas if you get lunch or a drink from the bar.
If it's the right season, this would be a great place to go diving as well.
The first time I stayed there, I stayed at this place called French Garden Resort. It's not much of a resort, but it was clean, friendly, close to the beach and close to the main road leading into Trincomalee. The 2nd time I went to Nilaveli, I stayed at Natraj Hotel which was also pretty basic, but clean, cheap and close to the beach!
Dambulla / Sigiriya
The only reason to stay in Dambulla is to see Sigiriya, Sri Lanka's primary attraction. It's a rock fortress that towers over everything around it. It's an incredible site and a real fun climb to the top!
There is also Pidurangala Rock, a fun and short hike right next to Sigiriya with great views! Most people climb Pidurangala for sunrise or sunset.
Also I would recommend going to the cave temples in Dambulla proper.
Accommodations: Colombo City Hostel has a location in Dambulla Dambulla City Hostel. This is also the area where I stayed for 3 weeks in Inamaluwa volunteering at a hostel, Jungle Vista. I'd be curious to see what Jungle Vista is like now. Since I was there, the owners had a falling out. The Australian owner, who's vision it was, is no longer a part of the hostel and the Sri Lankan owner took over the property, so i'm a bit bitter sweet about this place.
Train from Kandy to Ella
Though Kandy is a major city in Sri Lanka, I did not spend any time here other than to take the famous train ride from Kandy to Ella. Since you'll be basing yourself out of here, I'm sure you'll find plenty to explore. But the train ride from Kandy to Ella is not to be missed!
Hiking - Ella Rock & Little Adam's Peak - Both absolutely amazing hikes in this region. There's also another hike with a beautiful waterfall somewhere in the vicinity that I did not get to go to. Ella is a great place to spend a good amount of time if you have a chance. Ella Rock is an all day trek but is super super cool. Part of it goes along the railway tracks, then through small villages that are used to seeing travelers and can help point you in the right direction if you get lost.
Nine-Arch Bridge - a short walk from town to see this unique bridge
DO NOT STAY AT TOMORROWLAND HOSTEL - unless you want a party hostel. People raved about it (and raved in it), but I think the crowd was too young for me. The friends I was with and I were so desperate to get out of Tomorrowland and we had booked a couple nights at a place called 7th Hill Guesthouse. It was a bit out of town, but we loved the family who ran it. I remember them being super super helpful, they had their own tuk tuk and when available they would give us a lift into town and pick us up.
Situated between Kandy and Ella, I had to do a bit of back tracking to get here, but I didn't mind because of the nice train ride. Nuwara Eliya is known for their tea plantations, which are really fun to hike around. It's much less visited than Ella, but I actually found it to be quite quaint. There are many architectural structures built by the British giving it much of a British colonial feel. Victoria Park is great for an afternoon stroll and I really enjoyed their market in the center of town.
Horton Plains National Park - Nuwara Eliya is a good jumping off point for this beautiful National Park. It's a half day trip and definitely worth it!
Accommodations - I stayed at Hi Lanka Hostels which was a pretty basic accommodations, but I really enjoyed it because it felt more like I was staying in the a family's home. The owner, Hiran, was also kind enough to drive us around and show us different parts of Nuwara Eliya. He and his uncle also spent a rainy day teaching us to play Carrom, a Sri Lankan board game that you'll see a lot of locals playing.
A religious pilgrimage for Buddhist, an intense walk up thousands of stairs to a temple at the top of a mountain. This is seasonal due to the weather. I think the season ends by May, but double check with someone there. It's a highly spiritual experience. People start the climb at midnight to get there for sunrise. It sounds a little terrifying, but its a truly incredible experience.
Weligama - For me it was the perfect Beach to learn to surf. I spent a week here, going out every day, sometimes twice a day.
One cool place to eat in Weligama is along the water. You'll see all these vendors set up "restaurant" type establishments right on the beach where you can pick out fresh caught seafood and they'll grill it and serve it to you right on the beach. It's pretty fun for a group of people and the fish is soooooo fresh!
Accommodations in Weligama - Hangtime Hostel
This place was absolutely amazing. It's run by a bunch of Aussies. It's a surfer themed hostel with a bunch of beginner surfers staying there. Everyone was super encouraging and it was fun to see people in the water or up at the rooftop restaurant
Marissa - Since you're going to stay in Weligama, Marissa is a short bus/tuk tuk ride away if you want to check out another beachy area. I think Marissa is like the more touristy Weligama, but it's still nice and has a bit more stuff. Good for going out, maybe getting a massage...
Galle - I did a little day trip here too. This is like...the touristy and cultural area for old people. The fort is super cute area with lots of shops, restaurants, museums. It's more of historic area that's well worth checking out!
Other beaches that I didn't get to check out, but heard lots of great things about are - Arugam Bay in the East, Hikkaduwa in the southwest, Dikwella in the south (closer to Yala)
You should definitely do a safari too! Yala is the biggest Park. I did a safari closer Dambulla but that was only because the owner of Jungle Vista paid for it.
Something I also didn't get to do was visit one of the Geoffrey Bawa hotels. I meant to splurge on a night or two Heritance Kandalama hotel near Dambulla, but had too much fun at Jungle Vista.
If you really want to get zen, there's a very famous meditation retreat called Nilambe. I think the retreats are anywhere from 6-10 days where you live amongst people in complete silence for the entire time. I had a few friends that did this and I found it to be quite interesting, but it wasn't my cup of tea at the time. I would consider doing it if I went back.