We built a superconducting heat engine; it involved a liquid nitrogen bath, a powerful yoke magnet with mu metal, and a disk with high temperature superconductors placed around the outside of the disk. A horizontal axle goes through the center of the disk, the bottom of the disk is immersed in liquid nitrogen. Give the disk a little push to start it rotating, then it will continue to run by itself. The warm superconductor enters the liquid nitrogen, the magnetic field passes through the superconductor. The superconductor cools and begins to superconduct and the magnetic flux no longer passes through the superconductor, it now generates a force which rotates the disk. We also improved the project by adding diagnostics.
Another interesting undergrad project I did was to build a cosmic hodoscope. This involved scintillator paddles, dark boxes, optical fibers, photomultipliers, and timing electronics. When constructed one arranged two scintillator paddles such that if a cosmic particle passed through them it would cause light to bounce around the scintillator, into the optical fiber, to the photomultiplier tube, and to the electronics. If the electronics detected a signal from both scintillators within a given timing gate then we counted that as a cosmic particle that passed through both scintillators. One can get fancy with readouts, computers, and arrangement of the scintillators.