Lie to him. When you lay with him, tell him a lie. Tell him you had Chinese for lunch, when truthfully, you had a pizza.
Steal from him. A postage stamp. One first class stamp. Steal it from his day planner whilst he showers.
Humiliate him. Secure a plastic bucket of confetti above the front door so that when he walks in, the confetti showers over his head and his shirt. When he's standing there with little pieces of paper all over his body, point at him and laugh and say, "You look like such an asshole."
Make him shit when he doesn't want to. Fix him a sundae full of chopped up laxatives right before he leaves for choir practice. He'll sing off key and his fellow choir singers will find him disgusting and shun him. He will be too embarrassed to explain that it was only because he had to take such a mean shit. He'll drive home crying. He'll drive home crying.
Make him believe he lost something irreplaceable. You. Call him with a disguised voice and say, "I'm sorry to tell you that there's been an accident. She's dead. We can't find her body, but we know she's dead. There were hair fibers, all dead." Don't come home for eleven weeks.
Move him around while he's sleeping. If he's on the bed, put him on the couch. Don't leave a note.
Frustrate him. The next time he approaches you, say "Do we know each other?" He'll spend days trying to convince you that he is the man you've loved for three years now. Don't let on that you know. Just keep saying, "Um, I think I'd remember something like that. I'm calling the fuzz."
Blindfold him and spin him around and swing a bat at his face. This will kill him.
Order him to switch religions. Tell him, "It's splitsville unless you join me in the light of Lutheranism."
Cover him in ants. You'll need a lot of ants and he'll probably need to be tied up or drugged asleep to give you enough time to get each of those ants on him.
Hold him down on the bed and fart in his mouth.
Get fat. He'll be so disappointed. He hates fat women. Put on like 40 pounds and act like you don't think you're gross looking. He won't be able to tell you you are.
Kill yourself. He'll be so sad. He'll say, why? He'll wonder if there's anything he could have done. For the rest of his life, he'll wonder.
Happy Cruelty Day!