I'd say tripper or, as Delsyd, "user of psychedelics". Psychedelics are very rarely addictive, hence no need for a pejorative alarming term such as "dopehead" or "junkie" - however, they are no direct path to enlightenment either, and most trippers really do not aspire to really explore their minds (beyond that exploration which is inherent with taking the drug), in fact most people who aspire to explore their minds are probably not trippers but rather philosophers and psychologists; this is why I dislike "psychonaut" as well. Also, it sounds elitist and pretentious which goes against the most commonly observed values encountered in psychedelic trips, for many people.