Not much to see in the city and it takes about 1 hour by taxi. Also, given the security situation, I would not hang around the city centre. Suggest you go to Negombo which is 20 mins by taxi from the airport, a reasonable resort style town by the ocean. Taxi cost me 800 rupees (about $8) one way. I took a hotel room for the day - perfectly clean and reasonable cost about $40 but no doubt cheaper is available. If you want details PM me and I will try to find them.

Agree that the airport is not a good idea - supposed to be the best in the subcontinent but even so 27 hours is a long time!