When presented with a coursework brief to make a mechatronics project out of cardboard, I initially thought the cardboard restriction was very limiting. However, after some brainstorming, I realized that some systems might actually benefit from the structural properties of cardboard (stiff, very lightweight). I settled on attempting to make a cardboard balancing cube.
This project was incredibly satisfying for me, as I didn't know whether it was possible right up until the end. Watching everything come together after pushing through many many hours of prototyping, simulation, control design and tuning was very exciting; I was defiantly riding the edge of what was possible with the hardware I had.
More details are contained in a presentation poster I made for the submission: