My dd (16) is in 4-H, band, jazz band, piano lessons, American Rabbit Breeders, and soccer (9 out of 12 months).
She doesn't put the same effort into all of her extras but enjoys them all. My philosophy is that as long as she's having fun she doesn't have to be the best at all of them. The purpose is to keep her from "hanging out" , give her a sense of responsibility, and keep her active and managing her time.
My dh and I decided as long as she has a couple free days a week, that was enough for her "down" time.
Ds is 7 and he is in Tiger cubs and piano lessons. He plays soccer in Fall and Spring and t-ball in the summer. He is beginning to show rabbits and starting in Cloverbuds (4-H). Again, we try to make sure he has several free nights a week to do "nothing".
My dd friends who do NOTHING are always hanging out, they are lazy, some are smoking now, going to parties, riding around in cars, it's starting to become a disaster. dd used to complain that she couldn't hang out with them, now she says "you were right".
Hope this helps. It was kinda long!