Caribbean cruise advice

[MooBot3000]

I really am quite a n00b at all this. I'm interested in going on my first cruise with my gf (whose been on one small cruise from san diego down to a few mexican cities). We'd like to do a bigger cruise on a boat that has lots of fun things to do.

Which cruise lines are the best for this? How do you deal with alcohol on the trip? How long should I go for? What islands should I visit? Is the end result of the trip something that me and my girl will enjoy and remember?

Right now we're considering doing a standard 5-day Carnival trip from Miami to various islands.

Thanks for any advice 2+2.