If your imagination is stifled, not encouraged in any way, then it becomes stagnant and more difficult to access. As a parent it's a responsibility to let your children become who they are not who you expect them to be.
Let them run; roll down the hill; get their new shoes muddy; paint and draw and forget the mess until it's time for you all to clear up (yes, them too, it's important); bake cakes and not worry if they're inedible or all over the place; write silly stories; sing silly songs; make up words.
That's the jist of it all.
On another note, it's also worthy letting them fail, finding out that the world, that people, are sometimes mean and nasty. Grow from it, confident that they know how to deal with the situation, that they haven't been hidden away and protected to the point that they can't cope with reality.