I recently visited Sri Lanka on a 4-5 days' trip and thought of sharing my experience and feedback in case others might find it useful in their planning. To set the expectations right, ours was a pretty relaxed vacation limited to the western and southern coasts of Sri Lanka. The places we covered were Mirissa, Unawatuna and Colombo.
We landed in afternoon in Colombo and then spent the rest of the day relaxing after our travel. Next day morning, we left by car for Mirissa. What I learnt is that there are two routes to go from Colombo to Mirissa. One of them goes along the coast providing you a sea view almost througout and the other is via an expressway. On our way to Mirissa, we took the road by the sea and took the other route, which wasn't entirely an expressway though, on our way back. In hindsight, I'd say it may not be a bad idea to take the expressway bothways as it will save your travel time. Just ensure that taking the expressway does not mean letting go of any places that you would be interested in stopping at.
On our way to Mirissa, we stopped at a turtle hatchery, the Madu river and a Buddha statue built as a memorial for people who lost lives due to the tsunami some years ago. There are several turle hatcheries in the area and while it's debatable whether they've made a business out of it or are actually helping in conservation of endangered turtles, I think a visit to one of them would be an interesting thing to do. You can see the turtles from close quarters, touch them and even hold the baby turtles in your hand. It may be the norm, but the hatchery we went to had two types of tickets - a 1000 LKR one on which you could only see around and a 2000 LKR one on which, in addition to seeing around, you could also release a baby turtle into the sea.
At the Madu river, you have the option of taking a cruise that lasts about one and a half hours and takes you to several places like cinnamon island, snake island (you don't get to get off the boat, for obvious reasons!), prawn farm, a Buddhist temple etc. However, I did not find it great as you have to see several of these places from the boat itself which means that you don't see much. Also, at several places, the boat crosses under bridges where you must bend down to prevent bumping your head, especially if you're tall. The best part of the boat ride for me was the cruise through a really short mangrove tunnel.
Mirissa is most famous for the whale watching tours that go from here besides, of course, the regular water-based activities the place offers like snorkeling, diving, surfing and water sports. We didn't go for whale watching because of our own reasons, but if you are into nature and wildlife, you will probably not want to miss the spectacle which costs 50 USD per person. In terms of the chances of spotting a whale, what I gathered is that, if it's the season, then it may not be unlikely to spot a whale. Also, the tour operators stay put for a considerable amount of time in the waters to increase the chances of spotting. That is the reason that the tour may last up to 6 hours in total. In fact, I even read advertisements about aerial whale watching tours which you can explore further, if interested.
In Mirissa, we also visited coconut tree hill and parrot hill. The former is a small hill with coconut trees and gives you beautiful views of the sea from the top. The latter is another hill but I am not sure why it's named so as I was told that it does not have parrots. Nevertheless, this is a good place to snorkel or swim because of the crystal clear water and abundance of fish near the beach.
Our next stop was Unawatuna which is not very far from Mirissa. It falls in the Galle district and from here you can visit the Galle Fort area which has preserved its European look from the past. There are several good restaraunts in the Dutch Hospital building (which used to be a hospital in the past) and if you go to one on the first floor, then you can enjoy the sea view while having food. There are a lot of shops in this area where you can buy clothes, handicrafts, art items, gems etc and when it comes to art, I found the collection here to be much more unique and interesting than what I could find in the limited time (half a day) we spent shopping in Colombo.
While in Unawatuna, you can visit the jungle beach, which is accessible through a short trek in the jungle, and the peace pagoda which is atop the Rumassala hill which, in turn, has an interesting mythological connection dating back to the Ramayan period. The story is that the hill is a piece of a mountain containing medicinal plants that Lord Hanuman accidentally dropped while carrying the mountain. I am no expert but I read that the bio-diversity found on the hill is markedly different from the surrounding areas.
Finally, we returned to Colombo and did some sightseeing coupled with shopping. I liked the Gangaramaya temple for all the colours you find there, be it the statues or the paintings and also for the beautiful carvings on the wall facade beside the entrance. It had a special charm at dusk amidst all the lighting. Just across the road is another temple called Seema Malaka which is constructed over water. I'd suggest that Seema Malaka be visited before dark to get the best view.
We made this trip in the month of March during which time the weather was mostly hot during the day. If you are a non-vegetarian, then you will have plenty of food options to choose from. On the other hand, if you are a vegetarian, your options would be limited but with a little bit of searching around, you should be able to manage without much difficulty.