If you want to spend some time outdoors, visit Fairmount Park.
Behind the Philadelphia Museum of Art is Fairmount Park, one of the largest urban parks in the United States. Its 8,900 acres stretch along both sides of the Schuylkill River (pronounced skoo-kill) and offer a refreshing dose of natural beauty in this metropolitan area. It's much too large to see in a day, or even two, but the area behind the museum is a great place to have a picnic or stroll. You can also rent bikes or rollerblades and zip along the paved paths.
Enjoy lunch, the museum and the park in whatever order you choose. Unless you take a picnic lunch in the park, try London Grill or Rose Tattoo Café. Or venture a bit farther east to Marc Vetri's Alla Spina for excellent modern Italian pub fare.