The Franklin Institute is a splendid place to escape a rainy day. Here you can learn about all things science and have a bunch of fun at the same time.
This Center City display of scientific achievements is an interesting place to start your day of sightseeing. There's plenty to see in the vicinity, like Liberty Bell Center, Independence Hall, and University of Pennsylvania. Wander around at your leisure at The Franklin Institute before making tracks to these other fine sights.