What are some fun places in the philadelphia area where we can take young kids?

me and my girlfriend want to have fun with the kids(ages 4 and 1) at first i was thinking either rehoboth beach, willdwood, new jersey or the baltimore aquarium. however, i realized that those trips are not in the budget. due to other bills being due. what can we do for fun with the kids?What are some fun places in the philadelphia area where we can take young kids?
The Franklin Institute in Philly has science exhibits just for kids. You can also take them to Adventure Aquarium in Camden, NJ, which would be a cheaper alternative to going all the way to Baltimore. I think there are also some old navy and sailing ships on display along the Delaware River on the Philly side, which could be fun.
In Philadelphia, you can visit the Independence Hall where the Declaration of Independence was signed and the Liberty Bell. There is also the U.S Mint there to visit, and lots of monuments. There are also some nice landscapes and views with fields for nice picnics and cookouts.What are some fun places in the philadelphia area where we can take young kids?
tha projects aka the ghetto..i heard they have wonderful playgrounds all for free....