Got a bike? PG County has tons of green areas all accessible by bike trails that connect with the Anacostia River Trail and NW including Lake Artemesia and Patuxent Wildlife Research Center, as well as smaller parks closer in. Just turn on Google Maps bike overlay and look at all the green spaces and how they connect. If you want a comprehensive introduction, sign up for WABA's Cider Ride which is happening next week.

Of course you can walk these trails too, though it'll be harder to reach some of the farther-out areas.