If it's your first time in the general region, go to a Wegman's for groceries. It's probably the best grocery chain in the U.S. and no other store will feel the same afterwards.
Aside from that, there's the general tourist-y places - Reading Terminal Market is really cool to walk around in and go for lunch. The Franklin Institute is a kid-orientated science museum that's still neat for an adult, as long as you don't mind young children wandering around. King of Prussia Mall is less than hour (?) away and is absolutely huge - I think it's the second largest mall in the U.S. outside of Mall of America. Every time I've gone I had to break it up into two days because there's just so much to cover. Depending on the weather, you should also check out Eastern State Penitentiary. It's best on cold and rainy days, and it's amazing how eerie it can be even though it's in the middle of thse city.
Stay away from Intercourse, Kettle Village or pretty much any other Amish town. They're the biggest tourist traps and tend to be boring as sin. The only exception to this are the butchers you can find there, which, if you can transport it home, have the best meats and cheeses you can find.
Also, people will yell at you to get a cheesesteak, but get some lebanon bologna, some Tastykakes, and shoofly pie. Philly is great if you just want to be a fat fuck every day of the week.
Source: family is in Philly and we go every few years.