What is the best cruise for two 18 year olds?

My friend and I are 18 and looking into cruises for the summer before college. We are looking for both fun and relaxation. We are looking more for the Caribbean cruise and whatnot, and would like more nightlife than some cruise lines seem to offer. I've been on Royal Caribbean once before and it was relaxing but definitely meant more for little kids or older adults. Can anyone help?