Charlestown Community Center has 3 all-season tennis courts. They put up a bubble in the winter and take it away so you can play under the stars during the summer, with a great view of the harbor. The tennis is really cheap(I think $ 200 for the entire YEAR if you live outside of Boston, $ 100 in Boston)(this price may have changed). It’s not a well known in the tennis community, but a decent place to play for cheap! IT’s also a coummunity center with lots of other ammenities, I dont know what else they offer since I only went there for tennis.