There is no age limit. It is a family activity, which means that it is open to everyone. It is NOT part of the programming (kids in the Club/Lab at that time will not be attending), and there is typically an adult at each table with their children/grandchildren/whoever. Certainly, adults can attend without kids...but you'll probably put it in the "seen it once, won't do it again" category on future cruises as it is really oriented toward kids.
There are no other regular activities that require tickets at GS in advance. If you are on a holiday cruise, there is gingerbread house decorating. The Mickey 200 is limited to a certain number of families--you sign up by being there at the appointed time.