Calvert Cliffs State Park. It's about 1.5 hrs from DC and there is a nice hike through the woods to the beach. It's still warm enough to walk on the sand and look for shark's teeth fossils.

Check out KidFriendlyDC.com, my friend, a Capitol Hill mom, does an excellent job with it and updates events and excursion ideas at least twice a week.

Annapolis

We did a train ride up near thurmond a few weeks ago. They had bandits rob the train and a sheriff come and catch the bandits. Fun.

Dinosaurland. That place rocks. Kids lying with terrified faces under the descending foot of the TRex = priceless.

The Claude Moore colonial farm is worth a visit too-- off the GW parkway (fall festival in a couple weeks)

Glen Echo Park
Great Falls Park
Clemyjontri Park in the early part of the morning b4 it gets too crowded