Two things that helped wear out my kids at that age:
Rock climbing indoor and out - not only burns a ton of calories but requires mental stamina as well
DDR on the Wii - requires quick eye to foot coordination as they decipher arrows to foot patterns
As to underachieving- i wish someone would've advised me to go easy on my highgifted son when he was that age. He says the thing he hated the most was that so much more was expected of him, not just in what he learned but discipline level, work ethic,etc. just because a child is capable of doing multiplication at 6 doesn't mean he wants to work hard at academics. I'm sure I may be off kilter with much of the advice you receive, and I have no idea what will work best for your famiy, but I wish I'd done more to let him be a normal little boy.
