Meadowside Nature Center, it's just a quick 25 min drive from the college park/greenbelt area. Your kids will love it!
There is a room that is great for little kids to get down on the floor and play, filled with puzzles and books and enough blocks to build a decent size structure. And for the slightly older 5-7 age range there is a very cool magnifier microscope thing (in the same room) with a flat screen tv that projects everything under the lens, this entertained my kids for a long time. The walls are adorned with small aquariums housing snakes, toads and turtles. Outside is where the "awesome" is! In 3 large cages are birds that have been rescued and can no longer live out in the wild; a one eyed owl, a hawk, and the largest bald eagle I have ever seen. In another section of the nature center kids can dress up in pioneer clothes or climb in and pretend to paddle the Indian dugout canoe. My boys would agree that the best part was the cave in the Legacy of the Land Exhibit. Over and over again they went through the cave and down the small slide...some things just don't get old.
As an added bonus once the weather gets nicer there are apparently 8 miles of nature trails to enjoy. And the best part...it's all FREE!