Captiva and Sanibel islands have bike trails for casual riding, but be prepared for a lot of rec riders and other distractions and debris. It's nice and scenic, but don't expect to do a fast ride. Continue off the island onto Summerlin Rd. for a faster ride. Check into Calossa Riders Club for local rides. You might check google earth or virtual earth to look at the trails. I moved from that area around 5 yrs ago.
I have been over at Sanibel and Captiva. There are a fair number of people out there. Certain roads look as if they could be ridden easily. I suppose early in the morning would be OK. I doubt that there is much traffic then. As mentioned above, there are bike trails. I did notice that some of the riders were slow enough to get passed by a determined gopher tortoise.
2009 Cannondale Caad9-7/2009 BMC SLX01/2011 Marin Nail Trail 29er
I was only in Captiva for two days, got a good ride in on thursday, limited due to amount of time my wife wante to spend alone with my mother. It was fine, I could ride in the road with no problems and the cars were not very agressive like in Miami.