The Tamarind Tree is about 5 minutes from the airport.
They have both rooms and suites. Buffet is very nice.
Enclosed grounds.
Alternatives are the beach hotels in Negombo - perhaps 15-20 minutes away. Might be better if you like to wander a bit, as the airport area really doesn't have a lot to offer.
Downtown would probably take 45 minutes plus. Though if you are flying CX out (ie after midnight) that's not a real disadvantage. If you are looking for something clean in the bargain range try the Tropic Inn. Not that far from the Mt Lavinia Hotel already mentioned.
Happy wandering
Fred