I'd make them pee whiskey and shit skittles.
My girl would save me a lot of money, I'd have access to free Skittles and lots of guys would have funny, painful stories about being rushed to the ER after a night of rough, crazy sex resulted in a Skittle getting stuck in their pee holes.
Also, I'd make them immune to all diseases and their body's would be able to regenerate from any awful injuries...and they'd have a hormone that made violent, aggressive men lose their erections if they're ever sexually attacked.