I have never been to Ft. Myers, but I have been to Sanibel and Marco Island many times.
Sanibel has the old Florida feel, no street lights, no fast food places (the closest thing to one is DQ), there is a movie theater, a big place to rent bikes from on Periwinkle Way (most of the hotels/motels on Gulf Dr. have bikes for rent or free, though). It's a very relaxing atmosphere. There are many neat shops to check out. There is an ice cream shop called Pinocchio's (good homemade ice cream, but pricey). If you are interested in buying groceries, there Jerry's (no big name stores here, so no Publix).
I would definitely say that a car is a necessity. If you go to Sanibel Island, you have to check out Captiva. If you are looking for a relaxing vacation, this is the place.
If you want to know anything about Marco, let me know.