I think it is nice to have a multi-generational vacation! I wish my parents would do that with us! I have written a lot to a potential cruiser about Disney vs. Carnival and Disney vs. RCL - so I won't repeat it here. It was not up to our expectations for our family. Food, service were very poor. If you decide to go anyway, will Grandpa go to the Spa and the adult pool (it was very quiet there just like DCL promises - the only promise they lived up to actually!). We never got to be there for more than 15 minutes as they counselors did a poor job with our daughter and they kept paging us to come get her. Had I been able to sit there and relax and it was a nice adult spot with 2 nice jacuzzis.
The BINGO onboard was also really fun. I am not a bingo person, but this one of the few activities that was run well and with humor. The social staff kept it lively. Everyone showed up (I turned 40 on this cruise -but saw all ages).
There is small learning component and lecture series - they had the coach from the Mighty Ducks onboard. There is an ESPN sports bar. The promenade deck was relaxing for a walk, run or reading a book.
JUST REMEMBER it is a hectic ship with a young child and the food in the main dining rooms is poor - really poor.
Hope this helps.