I'd like to second the poster who said piano (or another musical instrument). DS4, also a mathy kid, is moving more quickly through Suzuki Book 1 than anyone might have imagined. However, if he learns a song quickly, it just means he moves on to the next, harder one. We've been learning a lot of good life lessons about daily practice, staying focused, not getting frustrated if the song isn't perfect right away, etc.
I also heard a presenter who spoke about the need for gifted kids to practice performance of some sort (music, theater, etc.) Often, they are so "in their own heads" that they have a hard time relating to an audience. This is problematic when they enter the work world and have to talk with supervisors and subordinates.
