Buses are cool
Buses are going all night between Colombo and Matara (in south) and are extremelly cheap. You’ll pay around 1.5 US$ to go with the air-con bus from Colombo to Unawatuna, or around 0.9 US$ for the regular ones. Transportation in Sri Lanka doesn’t offer a tourist scheme. Either you go with these buses, either you go with a private car (this is really expensive – around 70 US$ for the same distance). If you can’t get in drivers’ mood, and you afraid of their driving style (they seem not to care about anyone’s life, including theirselfs, but they have an internal, incomprehensible ration in what they are doing there), you can try the trains. Those are more confortable, more expensive, but not expensive. The problem is that you have a smaller number of options (there is only one train to Kandy and 3 to Unawatuna from Colombo).
