Having lived in that area for some time, I would not recommend Sanibel, Captiva or Marco Island for people who want to do more than visit the beach.
Ft. Myers Beach is much more active, and those other places are easy drives for short visits.
Sanibel is known for shelling, so if you like to do that you can spend some time on Periwinkle Beach.
Ding Darling is a nice place to spend some time walking, driving or riding a bike through, if you are into preserves and wild life...
Fort Myers Beach has quiet areas, and much more active areas. Sometimes people like to dismiss FMB as a lesser place because it's not as expensive and doesn't come across as exclusive. If you want exclusive, stay at Pointe Estero on FMB. You'll not be disappointed.
There is a Mucky Duck at FMB, too if that is a place you want to try.