This question varies depending on season and weather. Indoors–you can't go wrong with The Mutter Museum, aka "the museum of medical oddities." Outdoors–a stroll through the Italian Market, by 9th and Christian is always fun. Eat some nice lunch, grab a chocolate covered strawberry at Anthony's, and you're on your way.
This tip has the address "Mutter Museum-22nd and Chestnut, Italian Market-9th and Christian"