Places to see: Be sure to visit the John M. "Ding" Darling Preserve on Captiva Island. You can drive/walk or take a guided tour through this nature preserve.
Shelling expeditions: lots of private boats will take you to outlying islands for shelling excursions. These are usually 1/2-day leaving very early (pre-dawn) and returning around lunch time. Your hotel should have brochures and the local phone directory is chock full of names.
Eco-tours: At the Marina just before the Sanibel toll bridge/causeway, you can book eco-tours of the islands (1/2 day) and 3/4 day snorkeling tours.
Day-long Everglades tours: these are tours in oversized Jeeps; each person has their own seat and binoculars. You visit several places in the Everglades, ride an airboat, see wild life, tour swamp areas.
Restaurants: Our favorites on the islands (Sanibel and Captiva) are: Noopies (at Sundial Resort); Matzaluna's (reservations required, jacket and tie/dress required); Lazy Flamingo (great seafood); Timber's (if the seafood were any fresher, it'd still be swimming!); and finally, Cheeburger-Cheeburger (for the best cheeseburgers anywhere in FL!