GD - Where would you be flying from? That will make a huge difference in costs.
I'm actually doing some travel writing right now on beach destinations. If you're leaving from the West Coast, I'd personally recommend Tahiti. Not only is it stunningly beautiful, but there's a lot of culture, and literally thousands of cool things to do. You could explore underground lava tubes, participate in a walking over hot coals ceremony, watch Tahitian dance, shark diving...
If you're leaving from the East Coast, the Caymans have some pretty cool stuff to do, like snorkeling in Sting Ray City, but not too many all-inclusive options. The Bahamas, on the other hand, have tons of all-inclusive options, but not quite as many cool things to do. The Virgin Islands are pretty amazing, and they have some all-inclusive options. If you like eco-tourism, Maho Bay is awesome, though not all-inclusive. They do, however, have their own private beach.
ETA: Hurricane season might be a problem for Carribean destinations - A lot of the hotels in the Caymans have Hurricane gurantees though, and they'll refund or let you re-book your entire stay for free if you have to cancel or leave early.
Other good options include: Cozumel, Isla de las Mujeres, Brazil
i love the carribean. i've been to most of the islands and prefer St. John, St Maarten & St. Lucia. We went to St. Lucia for the first time last September for our honeymoon and the locals told us that they haven't experienced a hurricane in over 30 years b/c of the way they are located.
also, bahamas are great and you could always do mexico too. Fiji is on my personal list along with Bora Bora... however, the flight to Fiji is quite long...
For all inclusives check out Jamaica or the Dominican Republic. I went to DR with a girlfriend last summer and it was super relaxing and a gorgeous beach. We stayed at Barcelo Premium in Punta Cana (I picked it because they had a trapeze among the list of things to do. Not much to do outside of the resorts though.
Costa Rica is beautiful too and they also have a few all inclusives. My family stayed at the Barcelo there too in Tambor. The location was great and there were lots of things to do outside the resort.