You could look at one or more of the following:

- The sea turtle hatchery at Kosgoda.
- A river boat safari on the Madu river.
- Lunuganga estate, Bentota (Geoffrey Bawa's home and masterpiece).
- Brief garden (another masterpiece of landscaping, this one by Bawa's brother).
- The Kalutara temple and sacred bodhi tree.

If you had time to look at only one of the above, I'd recommend Lunuganga (especially if you have any interest in architecture or landscaping).