I've been an FTC (First Tech Challenge) coach going on 4 years now, and it's one of the most rewarding things I do.
This last season I also started assistant coaching an FLL (First Lego League) team. That team took the Inspiration Award at the Idaho state championships. They deserved it. Evidenced by the fact they've inspired me.
I have begun keeping notes, watching screencasts, brushing up on building with Technic sets, programming in in the EV3 environment powered by LabView, all so that I am better equipped to help them up the engineering learning curve.
As part of all that, I've been developing a simple, quick EV3 bot design to use for teaching. Something that when I want to teach about programming, we don't have to spend half an hour working on a bot to test their code out. But I also wanted something I could use to teach design principles with, as well.
To that end, I've developed RudyBot.
RudyBot isn't going to win any competitions - at least, not without a little help, but he's quick to build, compact,
has a low center of gravity, stable ground contact with low friction, uses a minimum of parts (less time lost to scrounging), is somewhat modular, solid, and easy to pull the EV3 brick in the case when you need to replace a depleted battery.
RudyBot is a little top-heavy. A fun exercise could involve taking this design and figuring out what changes could be made to lower the center of gravity.
He'll take a medium motor geared attachment like nobody's business and sports plenty of room to fit on other devices, such as a loop catcher. There's space up front for light sensors or an ultrasonic sensor (though, they do tend to crowd the medium motor access a bit) and off the top edges of the brick, there's a swell couple of spots where I'd recommend you snap a gyro sensor on. RudyBot wants to go places - and actually get where he intends to go!
I've used LDraw and Bricksmith for the Mac, plus LDView to turn the LDraw file into images and LPub to publish RudyBot as a PDF instruction set. RudyBot, true to his purpose can be built in 5 steps. Give him a go!
If you use RudyBot, drop me a line. I'd love to hear from you.
Update - 2/28/2015
Below is a photo of an alternate construction for RudyBot's chassis' runners. This construction sets the wheel back further with with a gear attached, and in front of it is a driver gear powered by the large motor. This design sacrifices some stability (due to the wheel offset) and might be a bit tippy on turns, but it offers the benefit of gearing. It's great for teaching the concepts of gearing up and gearing down a motor (just swap the little and large gears). The double angle beams also illustrate a means of meshing the black gears, which won't mesh if you're using axle holes in a straight beam.