You can bring Wine on board, but they will charge you a "corkage fee" (I think its $15??) They don't allow you to bring hard liquor on board. If you buy it in port, then they confiscate it when you come back on board and hold it until the last night for you. There are other topics that discuss methods of smuggling in booze, you can go read those.
As for excursions, book the popular ones before you go to ensure you get in. Sometimes there is room, but its better to be safe than sorry. I think if you change your mind you can cancel 24Hours in advance (but you may need to check on that for sure). I don't know what cruise you are going on, but if you are going to Grand Cayman and you want to do the stingray tour (which you do.. its great) that one you should book ahead of time.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007