The National Aquarium in Baltimore (Baltimore, MD)

The Baltimore Aquarium is one of our most favorite places in the area to visit. Yes, the entrance fees are high but if you become a member, it's completely worth it when you go 3-4 times/year and especially when you have guests from out of town visiting (use of your guest passes). They are also now affiliated with the National Aquarium in DC so you can get into both places (haven't been to the one in DC in many, many years but when I was a teacher I took my 1st graders there and thoroughly enjoyed it).
**Remember, NO STROLLERS allowed but they do offer backpack carriers (there is a stroller parking area inside where you pickup a backpack).
My biggest advice: In the summertime-Get there right away at opening to beat the biggest crowds. During the school year, go later in the afternoon after the field trips are done (1-2p.m.).
They currently have a great 4-D show of Dora/Diego.

LOVE the National Aquarium. What a wonderful place to visit in Baltimore. We live a tad outside the city so don't visit often, but it is worth it. I took our son their when he was close to 2 1/2. A key thing to note for parents is that strollers are not allowed. You check them when you get there and can pick up a backpack instead. My son still talks about the visit, but frankly at that age he had only 45 minutes in him. Get there early and expect to see only a portion of the place. A bit painful when you buy full tickets, but they do have family passes.