Precision Laser Measurement (Score 1)

A laser measurement experiment could cover a number of interesting theoretical subject areas: optics (diffraction), solid-state physics, and atomic level structure. Experimental areas such as error analysis, linear/nonlinear fitting, lab safety, etc. would definitely be applicable.

One measurement that immediately comes to mind is using single/double-slit diffraction to measure the width of a narrow object, such as a hair or a thin wire. You can place a strand of hair in the light's path and then use the measured distance between the interference fringes to interpolate the width of the hair.

In retrospect, I think this might be a bit basic, and might best be suited as an introductory experiment. You certainly wouldn't have to build anything, if that's what you're aiming for. I personally think that constructing an experiment, unless carefully designed to be robust, would certainly take more than 72 hours.

A few other sources come to mind for me. The American Journal of Physics, which is an pseudo-educational physics magazine, might be useful when looking for new experimental ideas. The lab class I took last year had a bunch of great experiments, some of which are/were fairly cheap to implement:
http://web.mit.edu/8.13/www/experiments.shtml