I like Christopher Columbus Park, nestled between the North End and Long Wharf. There's no beach, but it has a nice lawn and a fun playground for kids.

What about the 17 islands that comprise the Boston Harbor Island State Park, a 45-minute boat ride from Boston's Inner Harbor and the perfect place to work on your tan? More info at: http://www.mass.gov/dcr/parks/metroboston/harbor.htm. Closer to the city, there are some spots along the Charles River in the Back Bay area where you could unfurl that beach towel.