[RyanCahoon]

I think you need to make the activity much more structured. The fact that you're using "scrap" means that you're trying to make each build too unique. You could probably make an arduino build work, but LEGO / Vex IQ robots would be even better because of the standardized parts, ability to build without any tools, and easier reuse of parts.

Build a very simple drivebase from step-by-step picture instructions, hook up sensors, program using graphical language.

Have a simple game where they have to ferry balls back and forth between two points - one ball per trip. Whoever transfers the most balls without touching the robot (autonomous) or whoever can transfer the most balls in 60 seconds (teleop) wins. Keep a leaderboard going throughout the day.