South Seas is THE BEST!! I grew up going there (we usually did a split vacation Disney/South seas.) When are you planning on going? They are having an amazing deal through the end of September where you buy 3 nights get the 4th free and kids eat free plus a few discounts on other things. The best part about SS is that once you get there you don't have to leave if you don't want - there is a trolley that takes you around the resort and they have all the pools, shops, restaurants, golf, tennis, boating that you could need.
Other places we loved to visit while we were there were Ding Darling nature center, the Bubble Room for dinner & the Mucky Duck for dinner and watching the sunset. My mom also always made us do a little shopping on Sanibel - I think the place she liked was called Periwinkle Plaza.
We always stayed at the Beach condos which are at the farther end of the island from all of the main amenities (marina, tennis, golf, ice cream, pizza...) but we loved to take the trolley around. We also liked that we had our own pool and were right on the ocean so we didn't feel like we had to be at the more crowded main pool all the time. A newer trend there is to rent golf carts if you don't like to wait on the trolley - my friends just did it and told me the cheapest but nice place to rent them is right off property called YOLO - it's on google.
You have a little one so you could just get a hotel room at the hotel or you could get a 1 bedroom condo at any of the villas.
If you are a hotel snob you will LOVE South Seas Resort - I promise! We are going there for my 40th this summer - I haven't been in about 15 years and I can't wait!