If you cab it from and to the airport you should have time enough to visit The CN Tower and see some of downtown Toronto. Immigration lines and wait times are very minimal (Canadian side) US immigration lines are usually longer but if you hold an American passport I doubt you would be given any further scrutiny.

If you clear Canadian immigration and you have 3.5 hours left then I say you have enough time for a quick trip. Just make sure you have your boarding passes for your PHL flight prior to leaving the airport.