We've been on 2 DCL cruises without children and we've enjoyed them very much. Castaway Cay is something to experience, it's just wonderful. However, I don't think the adult pool on the ship is large enough, it was often too crowded to really do anything but stand in the water.
We did the 4 day last year. Me, DH, DS and DIL. We had a great time. We did dinner at Palo and also brunch on the sea day. My DIL and I enjoyed the spa on the sea day. There is so much for adults to do! Have fun.