Billy's certainly has the market on bike rentals. Every other biker I saw had a tag hanging off the back of their bike, letting you know where their bike rental came from! Also saw a lot of scooters zipping around with barely in control drivers.
There are 22 miles of bike paths (paved) on the island. Bikers can also ride the roads if they dare. I imagine this is a very popular way of getting around in winter when traffic is practically at a stand-still.
Rentals are right on Periwinkle main and is cheap...$10/day and fun way to explore most of the island. Bike paths (25 miles worth) are smoothly paved and there's galleries, shops and bars (while someone else shops) along the way. It's also a good way to check out other resorts for next time.
Although you probably could do without a car (shuttles from Ft. Myers airport) and have everything you need delivered to your condo (on-island grocerers will stock your shelves and fridge prior to your arrival, ask the rental agency), if the the weather turns sour, you can always pack up the gear and head off to where the sun is shining that day. Just watch the weather station, see where it's clear and go!! 2 hours to Tampa or for a longer day trip, head down to Key Largo..if you have to much fun, crash there for the night.
Rental rates at Hertz, Avis, Budget et al were sky high this year (Christmas season) and opted instead to use Double D. Very impressed by this limo/taxi service. The rates were very reasonable, drivers pleasant, Lincolns clean. All in all a very positive experience with this Sanibel Island based service. The phone number is 239.472.3458. They will meet you at the airport, take you shopping, or whatever you require. It was a money saving and enjoyable way to get around.