We always cruise with DS, & sometimes his friends, but since he became a teen, I am definitely experiencing the cruise sans kids!
I do see him occassionally because I insist he meet us for dinner most nights, lol, but he loved the club, the lab, and now the Stack, so he's off & running after the first day. (okay I see him sleeping!)
One thing I really, really enjoy is the Art of entertaining series for adults, especially the cooking demonstrations by the head chef. Unlike the other ships I've been on where this is a big group demo done in the atrium or theater, on DCL it has been in Studio Sea, more intimate with tables & chairs. The demo is followed by tasting everything & the food served for all those in attendance, as well as wine pairings served after explanations from the sommelier (sp?)! The recipes are available and have been fantastic.
Everyone has mentioned lots of the other things available for adults, and it's all good. I cruise a lot & people are surprised that DCL is our favorite, but I think they picture 100s of kids with no separation or structure, or they just don't realize the zillions of great things there are for kids & adults alike.