Philadelphia's museum of Art is a great place to spend a rainy afternoon. My daughter spent hours looking at the midevil armor and weaponry for a school project. There were lots of families in this section, with younger kids reverently gazing on the swords and armor.
After, we went through the European Art from 1850. The collection of Impressionist is comprehensive and delightful. I especially like the Cezanne's Mont St Victoire. My daughter enjoyed the Degas statue of the Little Dancer.
Don't try and visit the entire museum in one trip, its too much. Choose a few collections, and be sure to take in the view of the city from the top of the steps.
The only thing which could have been better, connect the parking lot to the museum so patrons don't have to get too wet!