Kitulgala is Sri Lanka's white water rafting central because of its easy access to the Kelani river. There are plenty of hotels and adventure groups who organize white water rafting (like Borderlands, whom we've reviewed), and we did ours through Sri Lanka Expeditions aka Kitulgala Adventures,
If you like water sports or you're just an adrenaline junkie, then Kitulgala is where you should go. In addition to white water rafting, you can also trek through the woods, slide down mini-waterfalls (both of which we tried), and go waterfall abseiling (use a harness to shimmy down big rocks).
Kitulgala is about a 2.5 hour drive from Colombo, on the Hatton Road. If you're not going to drive there, you can take the Hatton bus from the Pettah bus station and be there in about 3 hours.