Are you signed up to Travelzoo emails? If not, you should be. I get their daily emails and they have awesome trip ideas and inspirations. In fact, most of our trips (such as Tahiti and Peru) were found through their Top 20 deals.
Anyway, recently they posted an article about 15 Islands that you can travel to, without a passport! Some of the islands include: Guam, Puerto Rico, Amelia Island (Florida), but here is the full list: 15 Islands You need in Your life. No passport required.