Not from the east coast, but I’ll suggest some California areas:
- Newport Beach/Huntington Beach (a good "hub" location with tons to do in the local area, including great beaches)
- Coronado Island or San Diego downtown
- Avalon Catalina (if you want something a little different)
- Monterey for a few nights then cruise down the pacific coast highway, stopping in Pismo Beach
- Manhattan Beach or Santa Monica (great beaches, plus you've got places like the Getty Center, Hollywood, and Universal Studios a short drive away)
Those are some good spots with an 18 yo in tow. There are also plenty of couples type places that are a bit more remote (Cambria being one).