Thank you all who contribute to this party pranks series, as i told you, i cant pos all the ideas as i lost a file, but here you have:

get a trasvestite to make strip tease to the guy.
get him to get grabbed by somebody, ie. make a bet, then drop his pants off and give him a beer bath.
get him drunk, take away his wallet, credit cards etc. and put him on a train with only the exact amount of money so he can make a phone call.
get him drunk, when he pases away, put his leg in plaster, when he woke up, tell him that last nigt, he tried to dance in the table, he fell down and broke his leg.
put him an embarrasing temporarily tatoo on his back, so he cant see it until its too late.
when the cake arrives, voila, his wife jumps off the cake!
put a local anesthesic in his condoms.

Also check the part 2