Well, it won't likely be warm enough to swim. But I have to ask: if you don't know what's in Ocean City, why do you want to go? For a day trip to DC, you would need a car, but it would be a long day -- about 3 hour drive each direction.

You can go to the Maryland portion of Assateague National Park.

There is a great decoy museum near Salisbury which has gorgeous carved wooden birds etc (much more than just duck decoys). http://www.wardmuseum.org/

You could go to a Shore Birds game. http://www.milb.com/index.jsp?sid=t548

Plenty of golf that time of year.
PGN is good place for crabs. Very local vibe.

No trains to D.C. You might try Easton/St. Michaels area, another long drive from OC. Blackwater Wildlife
Refuge in Cambridge.

thanks so much, had heard about Assateague- sounds really interesting.

D.C. for a day trip is a no go. Too long of a drive unless you plan on tsaying overnight.

Ocean City is a typical beach place with typical boardwalk, clubs, bars, mini golf, etc.

We love the crabs at Captains Galley. Belly Busters has also had good crabs in the past.

There is a small place to the left of the road when driving into Assateague that rents kayaks and other water equipments (cash only). We've done it a few times and it is fun.

New and improved Assateague visitors center before you cross bridge into Assateague.

If you like to explore other towns for antiques, shopping, food, etc within a small driving distance: Salisbury, Berlin, Snow Hill, Milton, Rehoboth, Bethany, and more.

If you are into craft beer, you can visit Evolution (Salisbury), Burley Oak (Berlin), Dogfish Head (two locations in Delaware), 16 Mile (Georgetown), 3rd Wave (Delmar).

thank you jedivader-all the things you mentioned are of interest to us