Many of the same entertainers who do a "family" show early in the evening do an "adults only" show later. The big difference is that the language and subject matter in the later show are not necessarily appropriate for children. Your teen friends will not be allowed in these shows.
The shows are not rude or filthy--in fact, when there is audience participation, we were cautioned to "remember that this is a Disney ship!) They just contain a fair amount of sexual innuendo. Also, the night clubs are quite smoky after 9 pm. I've left shows several times because there was so much tobacco use going on.
I've read on these boards that one of the clubs might have become non-smoking 24/7, but can't confirm this. Will let you know after our August cruise!
There's actually a lot for adults to do. There is different types of music in different venues, some with dancing. There are the variety shows. THere are places to "just hang out." During the day, there are a couple of adult series--semi-educational, like the Art of Entertaining series. All of the "family" activities are open to groups of adults as well--some of these are hysterical.
The teens (which happen to be mine) in our group will be busy at The Stack. In any case, I would never even think of allowing them near any of the adult areas. 
