Mexico is absolutely gorgeous. If you're looking for a nice romantic vacation, head just south of Cancun to the Mayan Riviera. Cozumel is just across the bay-consider that too. The Mayan Riviera is nice- I went last year and stayed near Playa del Carmen, which is about a 45 minute drive south of Cancun. It's nice and peaceful, with many activities to do, and it's close enough to Cancun to go in for a day to party. Cancun is really crazy all times of the year, even when spring break isn't going on. The clubs are really crazy.
The Caribbean is absolutely gorgeous! Look into a few different places- Aruba, St Thomas, St. Martin, the Grand Caymans. They're all spectacular!
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