I'm not sure about rentals in the city because I live just outside and ride my own bike. But Kelly Drive is closed off on the weekends to motor vehicles and you can take a fantastic loop through the Art Museum area and boathouse row. You can also ride into Manayunk (which is considered Philadelphia but isn't 'downtown' per say) which is where the 'wall' is for the pro race, if you've ever seen it. It's a very bicycle populated area and there's a nice bike shop on Main Street in Manayunk as well as lots of trendy shops and places to eat. From that are you can also get on the Schuylkill River Trail which connects downtown, Manayunk, and several of the suburban neighborhoods. I can actually get on the trail at my house (45 miles NE of Philly) and take it all the way downtown!
Have a great trip
Laura
"Get a bicycle. You will not regret it, if you live." -- Mark Twain