A.T.
I would use a diaper cake for the decoration. You can either get the directions on-line to make it or have someone give it as a gift. Also I would have some "manly" food items as guys aren't much for cake only events.
As far as games, I am not big on them, but a fun one I have seen done is to get the guys to diaper a baby doll. Either time them doing it or make it a team race thing. It also gives them practice for the real thing....only a moving baby is more challenging! Just give a prize for the winner. Something men like....like a six pack or something...LOL! Don't sweat it... just have a nice fun party and maybe get a friend to help you with it. Good luck.