In a small bathroom, make the window unopenable/unbreakable and reinforce the door so it can't be kicked down. Plug the toilet in some non-obvious fashion, and fill the bowl with Tide-E-Bowl or similar blue toilet cleanser (not all that unusual, especially in a public restroom). Dump a couple cans of Clorox cleanser, or a whole jug of bleach, into the tank. When the target goes to use the bathroom, lock the door behind them.
Toilet cleanser and bleach react to make chlorine gas. I had a brief RL experience with this (accidentally used a scrubbrush with one cleanser that, unknown to me, had been used with the other and not fully rinsed), and took some permanent Con damage (scar tissue in the lungs). Nasty stuff.
You might have to set the toilet near a swimming pool or the like so the smell of chlorine (from the tankfull of bleach) doesn't give it away...