I like the idea of small challenges, but let me suggest something that might tie the semester together a bit more.
Start with a simple goal - one that can be completed in a 25 min period with the square bots (following a line? etc). The next week...add something to it to make it interesting. So on and so forth, until it becomes more complicated but gradually so that students aren't going insane about it. Now and then take breaks from progress on that to teach other lessons. A couple examples:
* 1 day of CAD or other training - could develop into more if there is interest
* A simple, non course related challenge, a la "who can make the slowest robot" (teaches gear ratios, stuff like that)
* Introducing one week where teams work separately on robots that complete different tasks, or on different arms and options, and then present them to others briefly in a demo
My main reason for suggesting this progression is that it gives students a feeling of progress, of having fun while watching what we learn unfold in front of our own eyes. Instead of starting from scratch every week, you give non-robotics kids a chance to gain skills slowly. Also, robotics team members will get a feel for introducing different topics and finding out what teaching styles work best. They can even write plans, or something similar, which is really useful for the season.
Sounds like you've got something great here
good luck,