I know it's not legal (I don't know that, actually, but at the very least it'd be trafficking), but if I could come up with an experiment on the ISS, it'd be to see if the effects of psilocybin and LSD have the same effects in space as they do on earth. Literally 'trip ship' experiments in mental spatial effects, visuals or lackthereof, and physiological experiments could be conducted in relative safety. The real innovation would be how NASA and their contractors would approach the experiment.

The most powerful unnatural feeling in the world (zero-G) and some of the most powerful chemicals to affect the brain.