I like the vent. idea. I wanted to do that using one of my kids as the doll. Problem was, by the time either of them were able to interact with me on that level, they were too big to carry in one arm like that.
A creepy marionette is another good one... The puppetmaster could be on stilts in some kind of circus garb.
Depending on your show theme, dirty miners, mechanics, hillbillies, bikers, thugs are all easy costumes to make. A lot of people are afraid of them.
To look meant danger, to smile meant death!