Attidiya Bird Sanctuary is one of the better nature breaks within reach of Colombo, a wetland refuge on the city's southern side. It is especially worthwhile for birders, who can find waterbirds and wetland species without leaving the metropolitan area entirely.
Go early, bring binoculars, and manage expectations: this is an urban wetland, not a pristine national park. Its value lies in seeing how much birdlife still survives at the edge of a dense, noisy capital.