I'm on the outskirts of Philly... You should probably do some of the basics such as the Liberty Bell, Independance Hall, etc... There is the Old City Walking tour for about $15... Go to Philadelphia Tours for a bunch of tours. They do have a Segway tour as well.

Another popular one is the Ducks... DUKW Amphibious vehicle... Ride The Ducks of Philadelphia

Depending on what you are looking for, there is the Franklin Institute (currently hosting a pirate exhibit) or the Philadelphia Museum of Art.

A good amount of things are definitely within walking distance... What hotel are you staying at? You can look at a map and mark off what is in a comfort level of walking.
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