What are some Fun Things To Do in the Baltimore Metro Area?

I am looking for something fun to do with my kids who are 8 & 4 years old.
I live right outside of Baltimore, MD and am willing to travel up to 3 hours.
I know that their are lots of things to do in DC (museums, monuments, etc....) but I'm sure how exciting that will be for a 4 year old.

Any ideas are greatly appreciated. Thank You!!!!

The inner harbor is a blast on weekends. Check out the bodyworks exhibit at the maryland science center. They are actual plasticized bodies that were donated to science so you can see the bodies inner workings including how the muscles work. Very cool! Port Discovery is hands on and designed especially for kids. The aquarium. Theres a battleship they can explore. Really theres a ton of stuff at the harbor. Plus you can see the O's play!

Balto Aquarium, Orioles game, Museum of Industry (trains and stuff if they like that.) You mentioned DC, be careful because they have so many parades on Constitution Ave. all the time, and traffic is not allowed into the city and parking is impossible. Annapolis is nice for the kids if they like the water, there are a lot of kid-friendly restaurants around the Naval Academy. Also, take them to BWI to watch the planes land! There's a playground and park right next to it.