Try swimming! There is a gym on Bainbridge, between 5th and th I believe (unfortunately the name escapes me at the moment) that has a pool and it's open 24 hours. Also, Aquafit I think...on Germantown Ave, across from Video Library (used to be Philly Total fitness) has a pool as I'm told. I'm actually thinking of joining that gym in a bit.

Swimming really gets the body working and it's fun to do....besides, as it gets hotter, a dip in the pool will only seem like more fun! :)