Some places let basically anyone be an MT (mental health technician, also called techs; these are different from the nurses and doctors) but since I moved, the local hospital only allows nursing students to be MTs. MTs get paid, though, so they're not volunteers.

All the art therapists I've seen needed certification, but some of the pet therapists and music therapists were volunteers. You really need to check with your local hospital, because every hospital has different rules.