Theodore Roosevelt Island is really gorgeous. There isn't too much to see beyond the sculpture, history-wise, but the area around there is very nice and because it's outside it's very kid-friendly. It's also off the beaten path so it can be less crowded than other memorials in the area.
This island, in the middle of the Potomac River, serves as an 88-acre respite from the big city just across the river and provides sanctuary for beavers, box turtles, opossums, gray squirrels, raccoons, rabbits, birds, and nonvenomous snakes.
Its 2.5 miles of walking trails make it something of an oasis—if you ignore the jet noise from nearby Ronald Reagan Washington National Airport. You can bird-watch and walk through swamp, marsh, and forest. The upper trail leads through woods filled with elms, tulip poplars, oaks, and red maples. In the center of the island is a 47-foot monument of Teddy Roosevelt. Call ahead or check out the Web site for the schedule for the ranger-led tours given throughout the year.