Hi sorry I didn't respond sooner. Camden Yards is beautiful. It is within walking distance to the Inner Harbor of Baltimore where there are tall ships, restaurants, shops and the Baltimore aquarium. Also, near the stadium is Babe Ruth's birthplace, which is the house he was born in and it's open as a small museum.
Citifield, where the Mets play, is another really great ballpark, lots to see and do, great food. It's in Queens, which is east of Manhattan, but it shares property with the US Open Tennis Courts. If you have any interest in that at all, the Open happens every August, you can probably get tickets for both events. Queens has a few museums and sights, but obviously Manhattan is the big draw if you were coming in to town.
The only other ballpark on this list that I've been to is National's Park, We were there last summer. We bought tickets off a guy in the street (yes I know, not always the best idea), but we got a great deal and really good seats in left field. It's in a nice area near a Metro station and Washington DC is just such a fantastic city, we were there for 3 days and barely saw what we wanted to see.
We are planning to go to Citizens Bank Park in Phili either in late June or in August. I'll let you know how that is afterward. We are hoping to hit the west coast next summer but that's just in the discussion phase at this point!