My wife and I enjoy doing an all-inclusive resort every year for our anniversary! We've been to Riviera Maya, MX, Punta Cana, DR and The Grand Exuma Island of the Bahamas all in the past 4yrs.
I'm not sure if your options are open, but probably the best beach we've ever seen is in Punta Cana, DR. We stayed at a Resort called The Majestic (upgraded to the Excellences Club). This resort was super nice, food and drinks were great and the beach was top notch! There isn't too much to do outside of the resort in Punta Cana. Probably the most fun we had outside of the resort was in Rivera Maya in Mexico (Mayan Ruins, "The Underworld" created by meteor that killed the Dinosaurs). We stayed in a resort called Valentin Imperial Maya. This resort was massive, but was really fun to explore and the beach has white sands and big waves. Food and drinks were decent for all-inclusive, but this wouldn't keep me from going back. Our last trip was to The Great Exuma Islands in the Bahamas. We stayed at the Sandals resort there. The people there are really friendly and and the food was the best of all the resorts. Drinks were a little weak, but the beach and the water was really nice!
All these range from around $2k-$4k depending on how many nights you want to stay. Sandals is a big "brand" name so expect to pay more going to their resorts.
From my past experience, Expedia and/or hotels.com has given us the best prices for our trips. If you go Sandals, use their website (prices seem to be the same everywhere) and they'll usually give you an upgrade or a room of your selection where you want on the resort.
Hope this helps and have fun!