While not as idyllic as the beaches along Sri Lanka's south coast, the sandy shores of Mount Lavinia—itself a southern suburb of Colombo—offer one major advantage: they lie just 8 km (5 miles) from the downtown area and are easily reachable on a day trip if your time is limited. You may want to avoid weekends, however, as that's when everyone comes to stroll and generally unwind. There are beaches closer to the center than this, but Mount Lavinia is wider and cleaner.
It also has the most extensive facilities, including plenty of eating and drinking options.
Mount Lavinia, Colombo, Sri Lanka