Challenges:
I used to belong to a community for SketchUp at SketchUCation.com. They used to and still have weekly contests. They would have rules like: you could only spend 2 hours to make something with the application for the contest. Now, you could spend as much time as you wanted to for planning or make a sketch on paper. The time limit with the application was good because it forced you to learn how to plan things out and translate your design to a finished piece. It also helped you organize your work and use the tools efficiently. If you found a weak spot in your abilities to render something with certain tools, it was a good thing because you could work on improving that later. You would submit a screenshot of your work after a week. Then, participants would vote. The winner would get to choose the topic for the next week's contest. I only participated in a few contests. I wasn't nearly as skilled as some of the artists there. But, they were always kind with their comments about how I could improve. And, like this website, seeing other people's work gives you ideas about how to use an application.
I went back to that website and looked up some of their contests. Here is a list of some of them. Maybe they can be used for the challenges here. As you can see, the topics can be very broad (childhood or sailing) or very narrow(a lamp or a chair).
Sailing, robot, cannon, Halloween, childhood, Dr. Seuss, crazy chair, kites, bridges, Lego, lamps, camping, fishing, underwater, jewelry, Hawaii, Valentine's Day.
Ken