Amusement and water park in Prince George's County, Maryland, just down the road from the University. Open April through October. More than 100 rides, shows and attractions.
» Six Flags website

About 2 hours south of Washington is Paramount’s Kings Dominion, a 400-acre theme park that’s home to more than 200 rides, shows and attractions. The park features one of the largest coaster collections on the East Coast, including the world’s first air-launched coaster.
» King's Dominion website

About 2 hours north of Washington is the park that chocolate built. HERSHEYPARK is home to over 60 rides and attractions, including 10 thrilling roller coasters, 6 drenching water rides, and over two dozen kiddie rides. Park includes live entertainment daily, a dolphin and sea lion show, games, food, and shopping.
» Hershey Park website

DC Pages lists some of the best theme parks in the area.
» DC Pages listing of theme parks