Cabo San Lucas, Mexico.
I've been to quite a few places around the world, but I don't think I've ever had as much fun anywhere else. The "do whatever you want" aspect of it is great. You can walk out of your hotel room with an alcoholic drink, right onto the beach, into a cab, on the streets, into one bar, and right back out. Weather is amazing, lots of activities like jet skiing, parasailing, booze cruises, etc. The nightlife is amazing. Downtown is one big party and cab rides are cheap especially since they are vans and you will most likely be sharing it with people. There are tons of restaurants there too with AMAZING food.
We did not do the all-inclusive thing, but became friends with some people who had it and they used it to it's fullest and then some. If you're planning on eating and drinking a shitload then it's definitely worth it. We went to Walmart for food and drinks during the day (which kept it cheap), and then at night we spent cash at the bars and nice restaurants so it wouldn't have made sense for us.
http://www.villadelpalmar-cabo.com/
That's the hotel we stayed at.