It's there, right on the edge of their current abilities that children are able to focus, practice, and eventually master a skill. (The same is true for adults btw). These activities offer just the right amount of challenge and interest for young children, and in turn build independence, grit, and real skills. Here's a few examples by age to get you started.
Here's a secret: if you offer activities that have real purpose to your child, she will remain busy and quiet longer than you thought possible.