At this point, I don't suggest coming straight out and saying "I am tired of money... the loser of this bet has to dress up like a slutty school girl for Halloween!"
Please use a little more tact.
Pretend you're fishing for an idea. "Mmm, maybe something a little embarrasing. Loser has to shave their head? No, no, no, too permanent".
"The loser has to sing?... No that's dumb"
"Halloween's coming up.... I bet you'd look good in heels!"
Now you're getting somewhere.