OP-Any navigators will give you an idea of all the different activities available. I've been on 10 cruises, all 4 ships, and the same types of things are offered. So don't feel like you have to have a Galveston Navigator to get a feel for what there is to do.
All 10 of our cruises were without kids. More than enough activity. Some family friendly, some adult only. Like mmouse37 I was worried about length of Panama Canal cruise due to 8 sea days (ended up being 9 for us due to a missed port) but I did not have to worry. In fact on that cruise I think I only went to the pool 3 times? We travel regularly with other adult only parties - its pretty common actually on
DCL.
How I typically fill my child free days on DCL:
Movies in the BV Theater
Brunch and Dinner at Palo
Coffee and Reading at Cove Cafe most days
More reading at Outlook Cafe
Mixology and other Tastings
Working out in the gym or walking on Deck 4
Trivia (at least once a day, sometimes twice)
Adult Entertaining Seminars (most sea days)
Variety acts at night
Spa (sometimes massage, sometimes just a pedicure)
Bingo (more mom than me but sometimes I tag along)
Character Visits
Shopping
Craft Sessions (orgami, tile coasters, 3d art)
Animation Classes (drawing disney characters)
DVC mixers
Castaway Club receptions
Play cards or a game in the Promenade Lounge
Get up early to catch sunrise
An hour or two at the pool reading
Evening shows in the WD Theater