They get to show what they think and what they can do. And they learn to share it with others. They learn to grow as others help them. They get to know what others think and do. They learn to colloborate.
It can be good to work with criterias but sometimes you have to do it without limitations. If you don't show the way you will be amazed of the results :-)
Use all kind of media so their is no limit for the kids not working traditionel... use sound, video, pictures...
Besides letting student find good ideas it is important that they have a real life focus and that they have an audience for their work.
Feed-up, feed-back and feed-forward can be build in as a part of students work in school and there is a lot of opportunities in finding people to help with the feedback.