Sometimes the cruise director will have blogs, complete with the newsletters for the day, so you can see what the entertainment looks like leading up to your cruise.
Depending on what line and ship you choose, there is plenty to do - poolside contests, dance clubs, casino, karoke, comedians, vegas style shows, plenty of bars, etc.
My husband, myself and a group of our friends did the Oasis of the Seas to celebrate all of the guys turning 30. Lots to do, you are never bored!