Sanibel Grill has a phenomenal grouper sandwich and that's saying a lot because I absolutely abhor fish. My husband forced me to try it and I absolutely loved it. They also have a decent burger and nachos.
We also love Cheeburger Cheeburger--they have amazing burgers, milkshakes galore and a cute diner atmosphere.
While Bubble Room is something to try once for the theming and atmosphere, I find it filthy dirty and expensive and have no desire to ever eat in there again. The ONLY reason we go there now is for take-out desserts which are out of this world and well worth the trip to Captiva. Try the red velvet and orange crunch--they are like nothing else.
For things to do, I HIGHLY recommend the Stars and Stripes dolphin cruise run out of Sanibel Marina. It takes you into the bay and dolphins follow the boat. You could reach out the back and almost touch them. Often the boat will stop somwhere in the bay where the dolphins are feeding and you can watch them up close and hear them breathe.
DH has been going there his entire life as his family owns a condo down there and I have been going for the past 11 so if you have any questions, we'd be happy to answer them for you. Sanibel is a pretty lax place so if you're looking for more entertainment, you might want to go to Fort Meyers Beach. If you like shopping, the Tanger Outlet Mall right off the island is fun--Coach, Claire's, Nike, Guess outlets, etc.