ftltravelcpl - good idea, but I dont think you can save money that way. I received some bar coupons as a gift on my last cruise. On the coupon itself, there is a limit on how much the drink can cost, preventing you from making any sort of gain. You can, however, buy cheaper drinks with it, so make sure you try to maximize the value of each coupon.
I actually made a game of it - I tried to find a drink that was as close as possible to the coupon limit, without going over, so I knew I was getting the best value.
RCCL - Adventure OTS (Honeymoon!)
CCL - Carnival Miracle
RCCL - Enchantment OTS