This assignment isn't incredibly fun, but it's satisfying, and it gives good basic SketchUp practice for the students. I give them 25 orthographic drawings of somewhat simple shapes, and they have to model them to scale in SketchUp. They get two points per object, but they're only required to make 15 objects. Any more than that and they get extra credit. Here are three examples of the drawing they get and the model they're supposed to create from it:
There are a million different ways to go about modeling something, and these challenges force the students to start seeing that.