Next to the road, Koggala Lake is alive with birdlife and dotted with islands, one of which features a Buddhist temple that attracts many visitors on poya (full moon) days, another that contains an interesting cinnamon plantation and also an (overpriced and touristy) herb-garden island.
You can take a 90-minute boat tour of the lake and islands for about Rs 4000 in a four-passenger motor boat. Besides the main sights, you'll stop at a small island village and see a lot of birds (hawks, herons, egrets etc). Look for the 'Bird Island Boat Tours' sign just east of the 132km post.