Floridian here, and Captiva and Sanibel are amazing, but they are not sugar sand beaches. They are shell beaches and are known for their shelling, but it's a little harder on the feet. Still, worth going - I recommend the Tween Waters Inn as they have a marina for fishing and a lovely beach. They are more casual than South Seas Plantation.
For sugar sand, Siesta Key and Ft. DeSoto North Beach are great, and there are lot of other things to do in Sarasota or St. Pete, respectively. I'm hitting St. John this summer vs. Florida, but you can find pretty beaches in FL.