I think there's a balance.
Answering your title alone, I think that children are at their most creative when allowed to BE creative, my field is Drama, and I have noticed that if you give NO structure at all, you get very little in the way of creative thinking, too much structure and the same result. The best results, enjoyment/creativity/devloping ideas usually come from a careful combo of structure, guidance and freedom/room for the child to move the stimulis forward themselves via discussion, play, group work or solo contemplation.
From your op I do agree that children can become self entertained if given the chance to do so, but that doesn't mean there's much wrong with out of school activities at all (though I do feel that the balance is off in many families, usually through the feeling that they ought to, there's HUGE competition amongst parents in our area - either to be completely self sufficient, or join as many groups as their are waking hours) with most of MY parenting I try and strike a balance between good things.
My children certainly benefit from the couple of clubs they belong to, from a social viewpoint and it's very good for them to mix with children who they don't go to school with. We also do not have weekends that have a set routine... we have few commitments (other than care of my FIL) at weekends and although we often go out for the day, we also have plenty of weekends where nothing is timetabled, and my children do entertain themselves happily once they understand I'm taking off my clown nose for the day!
Therefore OP I feel YABU for bolstering your good ways by putting down other perfectly good ways.
My children aren't keen on colouring in even though they can entertain themselves happily.