S.T.
The bell itself is pretty much nothing special to look at. there's a line of people and there's a bell with a crack on it. Independence Hall is another story - the artchitecture if beautiful and it's so very cool to think that this band of rebels met in that very room and signed the constitution. If the Parks guide is good he'll tell you the interesting stories about drawing he curtain during soem of the meetings so no one could see who was inside, about who sat in which chair, etc. If you haven't seen the movie, National Treasure you should see that first becuase that adds another dimension to looking at the buidlings, etc. (Just make sure your kids understand it's fiction.)
The Reading Terminal market is alot of fun and a great place for lunch that's an adventure and not too costly.
Franklin Institute (science museum) is a must-see for kids. It's on the other side of town BUT - it's a great way to spend a day. My kids were middle and elementary school at the time and while the libery bell bored them to death they loved the science museum.
I can't comment on any outdoor acitivities because when we were there (around 5 years ago I think) there was a record amount of snow on the ground and it was bitter cold so we did mostly indoor activities. There are also some great smaller museums we went to that were nice - and the riverside has a tall ship I believe and htere's an aquarium on the NJ side of the river that I'm told is awesome.
No matter what you'll have fun. With one kid in college and another in HS we don't get those get away times as much any more and I really miss them!