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!