This is such a sticky topic--activities for kids. I want my kids to have opportunities, I want them to feel they have had a chance to try out many different things, and I want them to find the surprises that lie therein.
For example, my son was in theater camp last 2 weeks and LOVED it. He has an excellent stage speaking voice. I never would have known that, nor would he have, if I hadn't signed him up.
My daughter did softball in the spring and was kind of terrible at first, but she worked at it and got better - that kind of strive to get better is so obvious with a sport where you can watch video clips of before/after or just know from how the ball connects that you've improved. Although she’s also improved during the school year in academic areas, it’s much more subtle, barring a few major milestones like learning to read. Suddenly being able to hit a ball when before you missed is such a tangible sign of mastery, getting better, improvement.
But, in contrast to this desire for many opportunities and to find their “bliss” (courtesy of Joseph Campbell and the Power of Myth. ) I want them to be reasonable little people with a lot of downtime.
I should take a moment to say that we don’t live in the kind of neighborhood where kids play together a lot. At least not my kids, it’s just not that kind of place, and while that’s a little disappointing, it’s something we’ve all accepted. When we schedule activities, we’re not taking time away from playing with neighborhood kids (though we are taking time away from swinging and digging in the dirt and riding bikes, just not with a gaggle of kids.)
So I evaluate, constantly, our roster of activities. I took piano lessons most of my life and think music education is really important. My son's into it - I can't cancel out of that. Gymnastics gives him a whole-body workout, and helps him manage a vestibular input thing that would otherwise be a problem, I think. Can't cancel that. He wants to play ball this year, try it out, see what being on a baseball team is like (and we attend a private school, so the opportunity to make new friends is pretty important.) My daughter is interested in the new daisy troupe at her school, and wants to do gymnastics as well (she has previously done piano and dance but she’s decided to take a break from both this year.)
Anyway, you see the slippery slope. Add in some activities after school, and it’s suddenly insane. And the list of things we’re NOT involved with is really long! They aren’t into scouting (though that might change for my daughter this year), they aren’t on the school’s chess team (which many of their friends are), they haven’t gotten involved with travel sports teams, are too young to play on the school sports teams, swim lessons are only in the summer.
Where’s the balance point? When is there too much going on, and not enough laying about? I have a feeling this is something we’re going to learn fast with a fourth grader and first grader this year. What have been your experiences? How do you ride this line? 
 Interestingly, in the friend's facebook status comments that inspired this post, one of her friends said they have chosen to do activities, but not to do school. Homeschooling, at least for that family, offered an opportunity to be free from the feeling that the kids are overscheduled.