At 12, DD found the lab to be "lame." Of course, it was her 9th time on the Magic, so she had done most of the activities. That said, the key to her enjoyment of the lab was always whether or not she made a friend in the first day or so.
Sign up for the Cruise Meets (find your thread and post) that you'd like to meet other 12 year olds. The kids can e-mail each other in advance, so boarding is like meeting old friends.
As to other activies--pool, all the deck 9 stuff, movies, the big shows, the family type shows (Mickey Mania, Walk the Plank, etc). There are family activities throughout the cruise (like making race cars, etc).
The funny thing was that the same kid who thought that painting animation cells during the lab programming was "lame" happily did just that in The Stack at age 13. Of course, it wasn't scheduled then--the teens have a list of activities they can do at any time, they just have to ask the counsellors for supplies. The group dynamic has a lot to do with happiness.
Also, point out that she won't be with 8 year olds--the 8-12 group is split into 8-9 and 10-12.