Can't speak much for Fort Myers, but my husband's family has been vacationing on Sanibel since you used to have to take a ferry to it and I've been going for the past 15 years.
Out of Sanibel Marina is the
"Stars and Stripes" motor launch which is an awesome boat ride which most often gets dolphins to follow the boat jumping out of the water so close to the back of the boat that you can almost reach out and touch them. We do this every year and absolutely love it.
For the best dessert on the planet, pick up the red velvet and orange crunch cake slices from the Bubble Room on Captiva island. I wouldn't recommend eating there as the food is overpriced and not worth it. The desserts are a different ball game entirely and can be carried out.
Another vote for Ding Darling. Definitely go early in the day, right before it closes, or during or right after a rain. This is when the animals are most active because it is not so hot. We've seen otters, raccoons, alligators, horseshoe crabs, pelicans, and various other native or migratory waterfowl. Make note, I believe they're closed on Fridays and, if you are a member of Ducks Unlimited or another conservation organization, you can get in for free.
Bike riding on Sanibel is a lot of fun. We like to ride to Bailey's General Store where they have cute touristy trinkets and a great bakery.
If you're in the mood for breakfast, we really enjoy Overeasy Café. And for the absolute best pizza and pizza crus, Island Pizza is fabulous. We order that in frequently on our annual trips. Cheeburger Cheeburger has fantastic burgers and milkshakes and Sanibel Grill has an out-of-this-world fried grouper sandwich (and this is coming from someone who hates fish!).
Have a great time and if you have any specific questions, please feel free to PM me.