I've been to Cancun and Aruba. Cancun is a party town for the most part, and there is a few interesting sites to see, but most are a full day's drive away. I have no desire to go back. Aruba is a small island, but interesting. It's kind of desert-like. There is a main tourist area, but the rest of the island is pretty relaxed. Cool bat caves.
Depending on where you are flying out of, I'd check out Puerto Rico. It's cheap to get to, US Currency, both very lush and desert-like depending on where you go. It also has incredible night life in Old San Juan and Condado, but can also be very tranquil outside of the city. Beautiful beaches, great culture. Lots to see, or just simply relax.