I'll have to agree with the others, just relax and enjoy your time off without kids.
You can go to the Carnival web-site and look through all the excursions and pick wisely, you don't need to spend a fortune, but its always nice to do something different and see some of the island at the same time.
What kinds of activities do you like, snorkeling, etc.?
NOt sure I would go through the hassle of bring beer onboard, you should be OK just purchasing a few a day, with meals, without going over your budget.
Just keep track of all your purchases while on the ship, they do add up quickly and have fun!