The obvious place is Negombo as it is only about 10 kms from the airport, but I don't rate the beach. It certainly isn't what you would call a tropical paradise. I think it is dirty and untidy and sea doesn't look very inviting. However, many people stay here for a day or two at the start and/or end of their trip, me included.
There are some nicer beaches north of Negombo, but accommodation is limited and access without your own transport is not so easy.
South of Colombo is Mt Lavinia, but again I don't find this what you would call a tropical beach experience. The accomodation area is just like a town suburb and the beach is over the railway track. Unless of course you can afford the Mt Lavinia hotel.
I think you have to go well south to get a decent beach and then you are a fair way from the airport. A taxi can do it in a few hours if you are prepared to pay.