Depending on how far you want to travel, I cannot recommend enough Charlottesville, VA. Probably a 3 hour drive from the DC metro area.
You can see Monticello (estate of Thomas Jefferson), the Blue Ridge Mountains / Skyline Drive, UVA, etc. It's a chill town.
Head north and check out Baltimore. The Inner Harbor area is safe and good for tourists. If you explore a bit, the city is full of history (Fort McHenry, the Shot Towers, etc.) Aquarium is real nice. Another cool place to check out. Probably 45 minutes to 1 hour north of DC, depending on traffic.
Not quite as far north as Baltimore is historic Ellicott City, MD. A bit of history, a nice main street to walk up and down, lots of shops to pop in and out of. Not nearly 'hours of entertainment', but its a cool little place and is 30 minutes north of DC.
If you want some time "on the beach", but not as far as Ocean City, MD, check out Sandy Point State Park, near Annapolis. It's on the bay, but nice. Head across the bay to Kent Island and check out the Kentmorr Restaurant. Good seafood and a fantastic atmosphere.
As for DC, check out the Smithonians (since they're free), but the Spy Museum is awesome and totally worth it. I haven't been in years, but the Newseum is great, too. If you're with kids, the National Children's Museum is awesome and I loved it as a kid and I love going there still with family. I'll repeat what other members have said - Stay on the Metro. The Metro is great and can get you most places in town for a nominal fee.
One last word - If you plan to drive out of town, be ready for traffic. I'm not talking Des Moines traffic, the beltway and surrounding highways are a mess.
I'm from the area, so feel free to PM me with any questions.