So far, we have limited our kids to 1 -2 extra-curricular activities at a time during the school year. For the girl, this means piano lessons and soccer. The boy has chosen baseball and soccer. (And they can walk to almost all of them - very little car transport necessary)
When they only have 1 -2 activities, we can still manage to eat dinner as a family 5 nights/week. I don't go too crazy from carting kids from activity to activity. They look forward to their activities and don't get too overtired or over scheduled. It works for us.
But...we are seemingly alone in this philosophy. All their friends are in 3, 4, or even 5 different activities. Daughter D. didn't do Girl Scouts, because of the 2-activity limit. Son D didn't play on a basketball team. And so on...you name it, they didn't get to do it.
It's uncomfortable to be the only family with a certain rule. And as the kids get older, we are finding ourselves in that position more and more. We have stricter rules about a lot of things (video games, bedtimes, etc.) and the kids are starting to notice that. (Did you know that my son is the ONLY six year old in the ENTIRE UNIVERSE who has to go to bed at 8 pm and is not allowed to get a Nintendo DS? Really, he is.)
I can totally see them adding activities as they get older. But now - they are 9 and 6. They still like to run over to the park and play a pick-up game of some made-up sport with whichever kids they can collect along the way. I don't want to fill up every afternoon with some scheduled activity. When will they have time for unbridled fun?
For now, I'll take the unstructured play over another activity. Husband D and I are both coaching soccer teams this spring and I can tell you - some of these kids are burned out. They don't want to go to a practice and play an organized game. They want to run around and be silly and have fun with their friends. This makes coaching a tad more challenging!
Of course, in 10 years, when my kids get rejected from their top college because of a lack of extra-curriculars....well, then they can prove me wrong.