There are so many activities for children today. From the time that they are born, you can enroll them in music classes, art classes, gymnastics, dance, sports, etc. There are computer games and television programs to teach skills. Truly, no matter what you want your child to be exposed to, there is a class for it. Parenting books are full of information on how to mazimize your child's potential and make sure that he or she is educationally stimulated.
There is nothing wrong with an activity or two to enrich a child's life. I have attended a weekly playgroup since my older son was 10 months old - it is as much an outlet for me to meet with other like-minded moms as it is enjoyable for my children. I also did music classes when they were small. As they got older, we switched to storytimes at the library which Isaac and I still go to while David is in school. David now participates in sports. Doing activities can be fun for both the child and the parent.
It is important to realize, however, that perhaps the most important activity that a child can engage in is play. Play is a child's work. While almost everything else can be learned later in life, an imagination can only be fully formed and developed in childhood. A child needs time to daydream. They need time to pretend both by themselves and with other children. They need paper, crayons, scissors and glue to draw pictures and make creations. Children are often so busy today going from scheduled activity to scheduled activity that they don't have the time to engage in this important work of imagination.
This morning before we had to bring David to school, my boys were busy playing a game of "pizza parlor." I don't know how it started but there they were, taking orders and tossing blankets in the air to make pizzas. They were so focused on what they were doing, they literally couldn't even hear me urging them to get dressed, so I got them dressed - pulling off their pjs and putting on their clothes for the day - while they continued with their game. We did manage to get out the door and they returned to "reality" but their temporary flight of imagination was beautiful to watch.
Children need time to just be children. Be sure to leave enough unscheduled time in their lives for them to do that.