Center City Philadelphia is fairly compact and walkable.  Wear comfortable shoes.  It's easy to find your way around; there are helpful signs all over the Center City area to direct tourist to the various historic sites and attractions.  

To get an overview of the historic sights and tourist icons, consider an open top double decker bus or trolley tour. Or, for those on a budget, buy a ticket on the Philly Phlash which takes you on a loop around the city, but without the narration.

Many attractions in the historic district are free or have a nominal charge.  Independence Hall is free but requires timed tickets. The Liberty Bell is also free. The Philadelphia Museum of Art is pay what you wish on Sundays. However, for museums that charge admission,  consider  The Philadelphia Pass may be a good deal, depending on how many attractions you are planning to see.  The Philadelphia Pass includes The Big Bus Company's Hop on Hop Off tour of the city,  as well as some dining discounts.  Many popular attractions are included in the Philadelphia Pass, such as : The Franklin Institute (though special exhibits and the IMAX incur an extra charge); the National Constitution Center; the Philadelphia Zoo; the Pennsylvania Academy of the Fine Arts; the Penn Museum;  the Academy of Natural Sciences; and Eastern State Penitentiary.  The Pass does not include the Please Touch Museum or the Philadelphia Museum of Art.

A great discount coupon for 5 Philadelphia museums can be found here: http://www.parkwaymuseumsdistrict.org. The coupon will bring you $2 off admission for up to 4 adults at: the Philadelphia Museum of Art, Pennsylvania Academy of the Fine Arts, Academy of Natural Sciences, Franklin Institute, and Eastern State Penitentiary. The site also has some very helpful Parkway Museums District visitor info.