Congrats! There's a fair amount of colleges nearby where students want to sublet their apartments for the summer. You can get to CHOP pretty easily by public transport (SEPTA). Some options are UPenn, Drexel, Temple, UofA, UofS, etc. A Google search of college sublets Philadelphia will get you on the right foot. If you have any questions, let me know! I go to school in Philly and am also an extern for the summer.