I like the All Inclusive resorts. It's a bit more upfront, but I tend to relax a bit more when I can eat wherever I want and constantly have a boat drink in my hand. I've only done this once, for our babymoon, and used Apple Vacations - I felt like we got a good price.
If you buy airfare separately, you might still have time to find a credit card with a sign up deal. Last year I got an AA card with 100k bonus miles, which could cover all the airfare.
Once your down there most of the resorts have a number of things to do, but depending on where in the Caribbean you head, there's can be lots of great activities in town that are inexpensive if you do them outside the tour companies. I really like Mexico, and from the Riviera Maya there's lots of day trips to see the Mayan ruins and hike. Makes for a fun day getting to wake up and do something cultural, followed by the beach and more boat drinks!