Okay, so maybe I'm crazy, but I managed to visit Universal Studios Islands of Adventures and the Everglades in the three days we had in Sanibel.
We also visited the J Ding Wild life refuge that is on Sanibel, which has lots of birds and we did see one alligator as we drove through. It has a nice visitor center with things for kids to do.
We tried to take the evening boat ride that goes through the J. Ding area, but the tide was too low and we couldn't get out. Please check with the company when booking that you can get out, as we were not offered a full refund once it was determined that the boat could not get out. We had to pay $4 each for a small aquarium talk that they did beforehand, which we would not have otherwise done. Considering the time we spent on the boat trying to leave and the time in line to get our money back, I thought we should have been offered a full refund.
Fondest memory: Universal Studios/Orlando is about four hours away, so we left at 5:00 a.m., got there by 9:00 a.m., and were back by about 11:00 p.m. If you have kids who might get antsy on a long beach trip and don't mind a drive, I'd recommend Islands of Adventure (make sure to get there early and do Spiderman first!). My kids loved this park. For those less adventurous, Busch Gardens in Tampa is two hours away and is supposed to be great.
The Everglades (Everglades City) to be exact is about an hour away. We took an airboat ride, a boat ride (saw manatees!!!), and a so-called safari ride where we saw alligators. We also visited an alligator farm where the kids got to hold alligators.