My favorite thing to do in the city, hands down, is to go roll around in one of the parks on a warm sunny day. Not that there is much else to do. Available Philadelphia activities generally include;
eating
drinking
shopping
going to shows
getting asked for your spare change
getting mugged for your spare change
jaywalking
eating again 'cause you're still bored.

However, when creativity strikes, or you are turning into a robot internet zombie while sitting bored at home (it happens) Here are some swell things to do in town.

Give out food to the homeless [it always makes me feel good about my self when I'm fucking everything else up]

Go to the soft pretzel factory by the Italian market at midnight. Skinny biker boys from all over Philly come out to grab cheap tasty pretzels, it is quite the sight to see.

Go thrift store hopping. Buffalo Exchange is my favorite, and after you recover from your depression over how expensive that cute dress is, skip on down to the Green Street Consignment where you can help out your wardrobe without hurting your wallet.

People watch in Rittenhouse square. Or in Washington square. Or on any patch of sunny grass/outdoor cafe/street corner in the nice parts of the city. Subways and trains are swell for this to, but be careful you don't come off as too much of a creeper.

Bum around used book stores. Cute one on Tenth between Locust and Spruce, my favorite however is near the corner of Second and Market.

Go espresso hopping. Alright, this counts as drinking, but is a good way to meet people. Find some of the smaller coffee shops in Philly (i.e. anything other than Starbucks) They will most likely be run by people who love the coffee bean, and love to chat about it. Spruce Street Espresso is small and friendly, Blue in Green will guarantee you a good conversation spiced with some sweet people watching action, and during a lull the guys at Brew Ha Ha are pretty entertaining. What's great about coffee shops is the conversational aspect. With the exception of the Last Drop, whose charm lies in their vehemence towards their customers, most baristas consider it part of their job to be chatty.

and thats it. there is nothing else to do in the city. at least that I can think of...
its been a long fucking day.