I have a six year old son and a four year daughter. My wife and I have spent a lot of time planning out the activities we want them involved in.
We both feel strongly that we want our kids involved in a minimum of one activity outside of school almost all the time, and that there are a couple of activities that are not optional.
For example, knowing how to swim could save their life one day. It is not optional and both our kids have been in swimming for over a year. Also, since we are Canadians we consider skating lessons, hockey school, and hockey not optional at least for a few years. Once the kids are old enough to decide for themselves, they will continue in Hockey. Our daughter will be starting in the fall and our son is going into his 3rd year.