Deliriants, inhalants other than nitrous (and, possibly ether), a whole ton of "legal highs" (like, "Neo-Doves" with weird, dirty stims), and anything terribly caustic or unpure.
But, shit, even being a small, quiet, white college boy, even the scourges of inner cities I would try in the right environment, for the right price, in the right form. I'd probably IV if I somehow came across a morphine ampoule. Crystal meth? Okay, if I have a milligram scale and don't have to be around the stereotypical "methhead" (no offense, hygienic tweakers). Shit, DMT sounds GREAT if I'm around people that wouldn't be fucking with me or asking me questions while out there. I'd plug something if it warranted it.