One of the largest on the west coast of Sri Lanka, or rather the third largest city, is Kalutara. Its name from Old Sinhalese can be deciphered as "black river". It is named after the Kalu Gangu River, which flows nearby.
It's a modern resort town with a beautiful beach, but a fairly quiet nightlife, so you won't be able to party until dawn here. However, Kalutara is guaranteed to have plenty of beautiful tropical scenery and a few interesting sights, such as the world's only Dagoba Buddhist temple and an elegant plantation mansion called Richmond Castle.
The resort of Kalutara on the map of Sri Lanka should be found a little south of Colombo. The distance from Kalutara to the center of Colombo is 44 km if you drive along the old coastal road. About 20 km south of Kalutara is another popular resort in Sri Lanka - Bentota.