I really really like amyl nitrite as a combination drug.
A combo I call the Magic Five - reliably delivering a transcendent experience, among myself as well as the friends I recommended it to - is LSD+MDMA+GHB+N2O+amyl nitrite.
You sniff the amyl just before inhaling the nitrous (while peaking on the LSD, MDMA, and GHB) ... the amyl finishes the combo off nicely, everything gets REALLY intense, body feels very warm, like lying in summer sunshine, perfect ecstasy, not a single thing that is imperfect or even conceivably so.