T.H.
Since they are so close together I do like the idea of giving him an event for his birthday. What about even some sort of indoor camp or karate class or something? Maybe your local community center has a basketball camp or something...it might be nice to get him active and see if he's into any of that stuff, I mean he doesn't have to be fanatic but he might enjoy it! I do like your aquarium and sporting event idea too, maybe even look into a membership to the aquarium if he likes that.
The 11 year olds I know really like Legos, video games, science projects and sports. Good luck!