CABBAGE beach is the one we have went to. Once on our honeymoon in 2002, and once with our kids in 2009 (they were 3 and 5 at the time).
It's a cab ride away, you will take one of the cabs headed to Atlantis-tell them you want Cabbage Beach and they will get you with others going there. Its about 4 Bucks per person EACH WAY. (they pack the cabs FULL)
They will let you out in pull around/drop off area that is near hotels, there is a path that goes down to the beach-don't worry you won't be alone, many tourists go to this beach.
When you are ready to head back, walk back up the path---there will be cabs waiting.
Its a very nice beach with a local feel. Local folks selling shirts and jewelry-if you are interested you can barter, if not tell them no thanks. They might come back just be polite and say no thanks again and they generally leave you alone.
There will also be locals renting chairs and umbrellas.
Locals also sell VERY strong rum drinks in coconuts. You can barter with them on this too
All that being said, I have walked the area right by the port, down to Senor Frogs and the straw market (before the new one was built)and back by myself with no fears. My husband and I walked around last year for a little bit and we were never approached or got any funny vibes from anyone.