My daughter is 15 months and we do Gymboree and Music Together once a week. She tags along to my son's playgroup once or twice a week as well, so she gets plenty of play time both structured and unstructured at the park. My son did the same activities when he was younger. Now, at 4.5, he does preschool (3 mornings a week), playgroup once or twice a week, swimming once a week, and either soccer of t-ball depending on the season. Most days, we have a structured morning and a free afternoon to do whatever we want.
I have noticed that if we don't do enough, the kids get restless and I get frustrated with them easily, which is why we get out almost every morning. On the flipside, my son gets cranky and annoyed when he is over scheduled and doesn't have enough time to just stay home and play with his toys. We definitely need the balance, which is why we tend to have busy mornings and quieter afternoons.