If you opt to day trip down, DC has so many options that it's almost impossible to narrow down. I'm a huge booster of almost every museum in and around the city - it's very hard to go wrong. However, if you want sheer nerd fun, the Spy Museum in DC and the National Cryptological Museum in Maryland are both a hoot. It's not cheerful, but the Holocaust museum is really the kind of place everyone should see once.

Really, the only thing I'd warn you off of is the Native American Museum. It's bland and generic, noteworthy mostly for its architecture and cafeteria.

If you're fond of just driving, heading west into lancaster county is beautiful. It's Amish country, and the novelty of passing horse drawn buggies with hazard signs can be pretty cool for a day.

It might be a bit late in the season, but you could also hit Delaware and Longwood Gardens which I have heard much of, but never actually visited. Rehobeth beach is a great place for beaches, fine dining and outlet shopping and also not too far away.

Honestly, Philly is a fantastic hub for getting to more good stuff than you can shake a stick at, so it's really more a question of what you want to see.

I had about an hour in Philadelphia on the way home from a business trip last year, but that was enough to take a peek at Thomas Jefferson's apartment and the Liberty Bell. Both very cool.