I just booked a trip there too! It wasn't clear to me from the website whether there were kids-only activities for 4-year-olds. The associated marine education program looked like it was for 6+, but there were some art programs and a swim school that didn't specify an age. (I have a 4yo, obviously.) Any of you veterans know if there are no-parent activities for kids that age?

Stuff I liked: exploring the island architecture in the old neighborhood south of Captiva Sunset, shelling at Blind Pass, Ding Darling National Wildlife Refuge, the Beach House bathing suit shop at Periwinkle Place (where there is also a small playground), margaritas at the Green Flash, the Sanibel Bean. We did not go out for dinner a lot - the food at Casa Ybel was pretty good but the service that night was dismal. We would take our stroller to Andy Rosse Lane and sit outside for lunch.
If you can, take a boat trip to Cayo Costa for a deserted island experience, preferably one where you stop at Cabbage Key for a cheeseburger. The area is great for fishing and kayaking.
Warning - Ding Darling's wildlife drive is closed to give the animals a break on Fridays. So if you go there on a Friday you'll have to walk. The tram tour would probably be fun with kids.

you bet! Even the nicest place to eat is casual. Report back how you liked it!

Thanks! Will my kids be fine in shorts and t-shirts?

There is an Andersons right before you cross the Sanibel causeway

Get one of those small nets for scooping up shells

I'd say dress code was Tommy Bahama bone fishing short over khaki or quick dry shorts for the men, cotton coverups and cotton sun dresses for ladies

Suntan lotion! The resort is very nice, and very new, since being essentially destroyed back in '08 by 4 passing hurricanes.
The shuttle bus will take you to the center of town, which is outside the resort. Drink away-cause you aren't driving. Maybe 4 or 5 restaurants there. You can also drive yourself back to Sanibel, which you have to travel thru, for wider choices for dining. Consider sailing school for yourself and kids, if you have them. The resorts rents out a zillion different water craft. Big game room, which when we were there, cost $20 for a full day of mind-numbing videos. If anyone fishes, bring the rod and reel. Snorkle? Bring that too. The place has both drinks and food, right on the beach. While not a big fan of Florida, my guess is, you will love it.

Going to Captiva/South Seas Resort next week for the first time. Very excited despite the mixed reviews on other threads. We are renting a car and I know I can stop at a store in Ft. Myers to buy stuff for the fridge in the room. Publix? Target? Is it super casual there or do I need any dressy clothes? Hoping for casual... Anything else? TIA!