While I'm sticking with my Playa Del Carmen advice I gave earlier, another good option would be New England. Rent a car, drive around, some of the prettiest country in the US. You could stop in Boston, NYC, or not?
If you decide on Playa I have friends that live there and own a great hotel.
Which Hotel? I will be down there in a month in a small hotel on the beach about 20 steps from the ocean.