Why Baltimore? you ask. The most amazing aquarium I've ever been to! The Baltimore National Aquarium on the Inner Harbor in Baltimore. When you get to the site, click Plan Your Visit>Discounts and Promotions to find deals on the tickets, which are pricey.
I usually buy my tickets for everything a la carte, but you can find some packages for hotel, meals, and VIP admission to the aquarium right here at the Visit Baltimore Harbor Magic Site.
Other destinations: the new science museum in Baltimore: The Maryland Science Center. This museum is also right on the water in the Inner Harbor. I booked my hotel on Priceline, and it allows you to select the Inner Harbor location.
I haven't been inside YET, but I was in Baltimore in December and went for a walk around the beautiful harbor. There was a long line outside the museum waiting for it to open on a sunny Sunday. Both the Aquarium and the Science Center have incredible deals on Friday nights! Starting at 5pm, they sell tickets for full admission for $8. Amazing! You can add on additional activities (like IMAX movies, 4-D theater, dolphin shows, etc.) for a small fee.
There are so many cool places to go with kids in Baltimore's Inner Harbor! But those are the two places I will take my kids in March.
The next day, I have friends who live in DC, so we're staying with them and then going to the FREE museums there:
The National Air and Space Museum is first on the list: it's a classic! I remember seeing my first IMAX film here in 1982...it was amazing! The Space Shuttle Discovery is arriving there in April, so we'll be too early for that.
Then, my older son is insisting on seeing the Hope Diamond at The Smithsonian National Museum of Natural History. The best part is: we can go in and see the diamond and leave for lunch without worrying about admission: the admission to the DC museums is free!