So when the bar monkey grabs it they throw it over their face

Or you could fill it with a sryup, but that made a lot of mess for the prankster as well as the victim

Probably doesn't work anymore, but on the old bars we would stack the drinks up above head height (for most anyway), and as they were reaching up to grab them they would tip a cup of water over themselves.

If it was busy enough you could get the same person more than once!