I've always been a huge fan of students creating 3D models on computers. It is a skill that combines mathematics, art, and design in a way that promotes plenty of problem solving along the way. Over the years I enjoyed watching students take off with this skill using Google Sketchup on PC's and MACs and ever since we rolled out Chromebooks I've been keeping an eye out for a rich, cloud-based 3D modeling tool.
The best one I've seen to date is Tinkercad. Simple, intuitive, and fun to learn; Tinkercad is a tool that many students will latch onto quickly.
Completely cloud-based, this tool gives a lot of the flexibility I had grown accustomed to within Google Sketchup. It allows students to start drawing from scratch, but also has a good number of premade 3D objects that students can use to play with dimension, space, grouping, coloring, etc.
Finally, we are in the process of pushing for more 3D printing capabilities in our district. Great news about Tinkercad- it allows exporting to all of the major 3D modeling standards for easy printing. And, as an added bonus, it allows the exporting to Minecraft-useable objects (how cool would it be for students to design their own 3D objects in school to import into their Minecraft worlds???).
I hope you give some kids time to tinker with Tinkercad on their Chromebooks- I promised you won't be disappointed in what some of them come up with!