There are musicals, variety shows, magicians, comedians, who whats to be a mouseketer every or something every night. It is usually done two times that evening so that each dinner seating will get a chance to see it. So if you have the early seating, you can see it after dinner. If you have the late seating, you see it before.
Also, there are lots of smaller things going on at all times-dueling pianos, 70's night etc. Check out the navigators for your particular cruise on Barb or Dave's site to get more info.
Have fun