They let him into a non-wellness when he was under 21? Wow...I didn't think they allowed that!
I'm really having trouble coming up with ideas. One of my friends turned 21 when we were down there, and we took her drinking around the world at Epcot. But that's not something I'd want to do with my parents, I don't think.
Bahama Breeze is always fun, though he might have gotten sick of it by now. That's where I had my first (legal) drink, with my parents.
Of course, taking him to a park with a birthday button, especially if it happens to be a park where lots of his friends are working that day, is always fun. (And, depending on your friends, can be pretty embarrassing sometimes, too!)
I'll keep thinking. I know there's good stuff, I just can't think of it right now!