Sinharaja Rain Forest is possible to visit as a daytrip from Galle. We rented a motor bike for 8 USD. Alternatively you can get a car with driver for around 80 USD. Each way took about 3h (access via Deniaya). The road until Deniaya is good, but traffic in Sri Lanka is a challenge (Buses, Tuktuks, etc.). The road to the entrance is a real adventure, due to bad condition. But with a motor bike you can drive to the entrance and avoid the 2km walk. The forest itself is pleasant, but don't expect to see too many animals (in fact I think I saw more animals along the streets and in the cities) but at least some lizards, snakes and the occasional monkey should be possible. I am sure the inscription is justified, but as visitor I cannot distinguish the difference between Sinharaja and other rainforests (Mexico, Malaysia) I visited, that aren't on the list.