The first thing I would do is contact the Sanibel-Captiva Chamber of Commerce. Their office is on Sanibel just after you cross the causeway on the right hand side. Call them, they are very helpful and can probably give you good advise on accommodations. Their link is below.
http://sanibel-captiva.org/
Sanibel is wide enough that when you drive through it you cannot see either the gulf or Pine Island Sound (bay). The Lighthouse beach is located on the south end of the island and has restroom facilities and parking is paid by the hour. Since it faces towards the south it catches a lot of shells, if you are into that sort of thing. Watch out for you back if you do this, you might pay later. Ask me how I know! There are a lot of restaurants on Periwinkle Way (main drag), one of my favorites is the Lazy Flamingo. More of a bar restaurant, but has a very good grouper sandwich.
Captiva is much smaller than Sanibel. You have to drive through Sanibel to the north to get to Captiva. Restaurants that I like are the Mucky Duck. Beachfront restaurant with limited parking. Does not take reservations. Food is good but it is small. The Bubble Room is a very eclectic restaurant with great food and fabulous desserts. A little upscale and does not take reservations and limited parking. Key Lime Bistro, good food, upscale and does not take reservations and no parking. The Green Flash, bayfront dining, decent food with decent parking. Remember, most of these restaurants do not take reservations and this is the height of the season, so they are busy. Be prepared to wait to be seated unless you go early.
I hope this helps!
Rich