Your best bet will be: City Bikes (305) 682-8889
20335 Biscayne Blvd They are located in Aventura right by the Aventura Mall, ask for Jaime when you call, they rent road bikes, Treks, I believe. There is a small group that ride laps around the golf course behind the mall durning the weekdays, each loop is 5K, it is also good for running. On Saturday morning, they leave from the shop and ride up to Ft Lauderdale.
if you have any other questions, let me know PS..Be very carefull riding down here.

Ride on key biscayne and give AIA a shot as it runs through South Beach...good sight seeing. I know of Elite bikes and Mack cycle...there's a ton down there.

make sure to pick up some body armor while you are at it :)

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Ride Scoozy Electric Bicycles
http://www.RideScoozy.com