I am in Naples right now! I second the votes for Sanibel, or Captiva. I love Captiva. From most anywhere in Ft Myers beach to Sanibel is no more than 20 minutes, Captiva another 10-15 minutes. Go to the Mucky Duck in Captiva for an OK meal, but the best happy hour/sunset crowd around. Or tour South Seas resort, a beautiful spot that we can't wait to get back to.
Or come down this way to Naples, probably about a 40 minute drive at most. Come to shop on 5th and 3rd, and dine at any one of a bunch of GREAT restaurants. Though it's not the best restaurant, we always hit Tommy Bahamas on 3rd; the food is very good, the drinks are fantastic and the atmosphere is great.
Naples is beautiful.