As an old fogie myself (so says my wife) I think Ambergris Caye, and Caulker, too, are great places. There's some nightlife if you want it, but it doesn't interfere with one's quiet enjoyment of a good book or a rum and tonic.
Caribbean Villas would be good choice, and there are a number of other places, too, in a similar vein. Some of the small condotels are wonderful, and they offer a lot of space and value for the money.
My new guidebook to Ambergris Caye will be out shortly -- you might look for it starting around the end of this month.