My DS, now 4 started at a very much unstructured "playgroup" type of environment. He was very shy at the time and he was absolutely lost. He has always been a boy who has needed firm guidance and needs to know what is going to happen next. Even now at weekends, the first thing he will say when he wakes up is "What are the plans for today?" Anyway..... Despite the staff saying "do you want to do some painting?" he'd just automatically say no, so it was just left. He never settled there anyway so I took him out.
At his next pre-school it was much more structured. They had periods of free play and then periods where there was a specific activiity that the whole class was involved in. eg singing/music lesson, "keep fit". At the time, he waas very shy and if he could have opted out of the singing session he probably would have. The teacher said that he would never look her in the eye when he sang as he was just too shy doing it! However......as time went on, he got used to doing stuff he wouldn't automatically have chosen to do himself, and grew to love it and his confidence really grew because of it. It really expanded his interests, and he LOVED performing in his end of year show.
The crucial point I think, was that the pre-school never FORCED them to join in with an activity if they really didn't want to, they were just very much encouraged to join in, however, they were expected not to disrupt the activity and instead had to sit quietly and observe the others. Doing this, my son got to realise from the children's faces that certain activities were actually fun to do, and this encouraged him to join in with things, shyly at first, and then with gusto for most things!
Now he has started school he has settled in so well, is used to a structure, and periods of having to concentrate at the times the teacher tells you to. He is confident at most things, and loves to try new things, and I just think that if he'd had the chance to opt out of things he wasn't keen on at pre-school, he just might have got used to that way of doing things and also never broadened his horizons so to speak.