My husband and I did a 3 day tour of Philly last year. I loved it and would love to go back!
I found the tourist things to be walkable and grouped together. The city has great signs on every corner pointing out the things going down that street.
If you are going to see the Liberty Bell get in line early! The line starts early and gets long. If you want to see the Old State House, get timed tickets across the street at the Visitors Center.
We loved the Constution Museum. I think it is easily one of the best hands on educational mueums I've ever been into. It explains the process of the government very well and I didn't find it boring for us adults at all.
There is a little place across from the Liberty Bell that has a 360 movie screen that lasts maybe 15 minutes I think. It's a nice break if you need in out of the heat or rain or just to take a breather. However it really is just a Go America type of movie but they had some cute stuff if you need souvenirs.
My husband and I try to tour alot of cities. I of course try to have a touring plan but things never happen the way I plan. We always buy a visitor pass for a city.
For Philly we used:
http://www.philadelphiapass.com
It worked great and we could use it for several things. We did a duck tour, trolley tour, used it to get into all the traditional touristy things and because we needed a break from the weather used it for some things that wasn't on our touring plan.
One of my favorite things not on the traditional tourist list is a tile mosiac inside an old building that I want to say is Harper's Magazine. I hope someone else can give further description. I want to say it was called the hidden garden or something similar. We found it after talking to a libriarian.