OK .. here's a couple ideas - its after 5 on Friday, so be gentle....
Get some photos of naked guys or guys in their undies or drawings (a little less $$ on the printing ink costs)
Then get pix of clothes to put on them. YOu know, like the old paper dolls with the clothes that stay on w/ tabs.
Whoever you send them to has to pull the clothes off to get to all the details about the party.
Or you could make POP-UP cards where when you open them, its a naked guy w/ POP-UP dick & balls!
Or you could just do the standard "You are invited to:"...