One on Three
Fill three glasses with coke, water, beer or stuff like that. All the members of the cast have to spit into two of those glasses and (who wants to) sneeze in them two... to make it really disgusting! Then, one blindfolded member of the cast has the three glasses in front of him, if he will be lucky he will drink the normal glass with the normal fluid in it, if he's out of luck he will drink one of the shitty glasses with all that poo in it... The guy must drink it all until the glass is empty....
There's a variation if you wanna be real bastards:
Fill all the glasses with sneezes and spits (there will be also coke, water or whatever you want), make the guy (who's gonna drink) believe that only one of the three glasses is filled with that shit... he's obviously gonna drink that poo!