Huffing glue or gasoline or things like that. I did computer duster a few times back in the day, but I would never repeat that either. Only inhalants I'd do now are ether, Nitrous, and poppers. Also, any deleriant like Datura is a no-go for me. Besides that there isn't much I won't do, I've shot heroin, coke, meth, k, all kinds of pills, smoked crack, black tar heroin, and PCP.
Edit: Id also never do PMA or take piperazines intentionally, they were awful when I got rolls cut with them.