Most cabanas are for 6 people. Only one person needs to do the rental. Pay for it and collect the funding from the others off-line. Cabanas get lunch deliverd to the cabana but you are welcome to check out the other offerings on the island (The food is a lot more diverse and better than it was in 2017).
As far as what to do, you can hang out at the cabana (I'd recommend south beach as it is quiet and tranquil). Use the pools, snorkel, eat, eat, oh, did I say eat, drink (your shipboard drink package works here), If you are into water parks, you can purchase a pass for, imho a fortune, you can take the baloon ride, you can zip-line. it's totally up to what you want to do (or not) during your day there.