I have had the chance to spend the last month working in a computer lab with elementary students. A project that I had the cycle threes (grades 5 and 6) start working on is using SketchUp to design their dream room. When finished, they are going to present a "virtual tour" of their room, showing off what they chose and why they chose it.
Obviously, they needed an introduction to SketchUp before they would be able to complete the assignment. I used the SketchUp support website http://sketchup.google.com/support to introduce the tools and techniques they needed to use to navigate and work in three dimensions.
Once they had a chance to experiment, I gave them the assignment. They had to use models I provided to furnish the room - I used models available in the 3D warehouse. I have tried to make the assignment as open-ended as possible, but with real world constraints. For example, the students must work within the walls of the room provided (because they can't move walls in their room in real life), but they don't have to build the walls or install doors or windows themselves...
The assignment is a four-page document (2 double-sided pages), the empty room is available on the 3D warehouse, and the evaluation criteria is a single page. I have even included an example of a completed room with all the requirements. Please feel free to modify anything as you see fit, and enjoy!