Cruise Fanatic has a great suggestion. Nachi has a swim up bar and only allows 100 guests. You do have to have reservations. We stopped there on the scooter because it was raining. They didn't make us leave, but they didn't want us there. We stayed over by the restrooms under the walk way until the rain ended, then left. But it was a really nice facility.
I, personally, enjoy Chankanaab and think the entrance fee is a little too high. However, the area itself is very nice. There is a little zoo and some nice gardens, there are shops with about the same prices as in town, and we've always had really great service there. If you print off the coupons, it will save you some. The cab fare will be a couple of dollars less than Paradise and probably 4 or 5 dollars less than Nachi.
We went all the way down to the Reggae Beach, then started up the other side before the rains came again. For a totally desolate beach, that would be the place. I don't think there were more than 8 people there. However, I'll bet the cab fare there would be $30-$40. There was a bar/restaurant, no chairs on the beach, and a couple of shops. Unfortunately, no live music, though.