I was thinking about a challenge based on those photos where people create imaginative scenes with eggs. I think a mini challenge like this will be suitable for all skills levels since the challenge is really about imagination, only rule is (and I haven't thought it out completely) that you cannot add anything to your scene that isn't possible in reality because what makes these scenes so cool is the fact that everyday day items are used so creatively. So if you're a beginner, you'll still learn about modeling, texturing, shading/lighting and composition and if you're experienced well, you can have a bit of fun letting your imagination run wild.