Like your DH I'm sure that mine will love it as soon as we are actually on the cruise! He wasn't so sure about WDW before his first visit either and now he's obsessed!
Hey, my DH doesn't even like WDW very much, so you have a big advantage there! The characters, pirate's night, the sailing away party, etc., are all very exciting and feel like shows in Disney parks. My DH doesn't love the characters, loud music, or hype in general, so we steered clear of a lot of that stuff. But if I were with others who enjoyed Disney--or kids!--I think a lot of that stuff would be fun and would add another layer of must-do's.
Our DD is very excited for the AquaDuck, she saw it on tv and has been talking of nothing else. So we will definitely have to find low wait times so she can try it a few times. Hoping to let her do it several times on the day we board.

We will definitely have to try the detective agency, that sounds fun!
Yes, the day you board and the days in port were definitely the better times to give it a try! We did it towards the end of our day at
Castaway Cay and the wait was only like, 15 minutes. It's a tame water slide, but it's fun and the marvel of doing something so advanced on a cruise ship--five minutes away from your room!--adds a layer of excitement I can't really fully explain. Another thing we enjoyed was the arcade--it's small but it wasn't crowded at all and we had fun playing air hockey and Guitar Hero. My DH is a big kid in a lot of ways (he just has no nostalgia for Disney since it wasn't a part of his childhood), so a lot of the simple things like that were big hits.
I think the best way to put my finger on the "busy-ness" of a
Disney cruise (especially the first one, as you're enjoying the newness of the ship) is that everything is occurring only minutes away from where you sleep. At WDW you might be likely to go back to the room after dinner and call it a night--or take a break for a few hours in the afternoon. You CAN do that on the ship, but you could ALSO go and run over to check out something new that is happening. So even though the activities are short or small, it feels like you're out of your room a lot. Every landing on every staircase has beautiful Disney artwork, and so just around every corner it feels like you're discovering something new and cool. And because everything is so close to where you sleep, the layer of stress that comes with getting back and forth is gone, so it just feels very relaxing. Just soaking up the sun and the ocean breeze feels like a small activity/adventure, even if you weren't ever the kind of person who thought that would be a source of entertainment.
Did you see the shows? Was there one that was a "must see"?
You know, we had late dining and before that we just weren't motivated to go see the shows. We were showering and relaxing after our days in port. I mean, I wanted to see them, but knowing DH probably wouldn't enjoy it anyway I didn't push for them. Everyone says they are incredible though, so really don't miss them!! We did see the ventriloquist on board and we both really enjoyed her shows.
One tip I will give is that, as first time cruisers, we were a little confused and frustrated by the opening hours at Cabanas. We loved the meals that were served there, but we always felt like we were just missing the end of lunch. With late dining, the stretch between getting back on the boat at port and our 8pm dinner always felt very long to us. The places by the pool were always open, but as vegetarians we definitely got kind of sick of the few options we had there. (However, I did discover the carrot and celery sticks at the toppings bar halfway through and enjoyed those with ranch dressing! It was a nice alternative to pizza.) In the future we decided that we'd definitely pay closer attention to the opening hours at Cabanas and maybe try to plan ahead for snacks in the afternoon by saving some stuff in our room.