Don't even think of renting a car!
Driving in Sri Lanka is unlike anything you have ever experienced. It makes New York, Paris London or Los Angeles seem like a walk in the park. It is manic and a major experience but... There are only two lane highways and each side are cars, buses, tuk tuks, pedestrians, dogs and elephants. The buses are the worst, if they can't get through on their side of their road they simply use the other side. Use of the horn virtually continuously is mandatory it seems. Get a good driver, preferably with a minibus, sit back and enjoy. Its cheap and its fun. Getting anywhere in Sri Lanka takes hours but there is so much to see on every road its worth it.