What is the public domain? How do you find resources in the public domain? How do you know what you can do with them? How do you release your own content so that others can access and remix it?
The challenge would involve three stages
1. Find some work in the public domain; (pictures, music, written work)
2. Remix it - use the work you found as inspiration for your own creation.
3. Release it - publish your work so that others can use it in the same way, using an open license.