I would sneak into their parents' bedroom and dial the international operator. The time difference was perfect--it was nighttime in the Central time zone, but morning in Oslo. Of course, my reaching out and touching Norway left the party host with a staggering and mysterious phone bill--and an ass busting when his parents got back into town.
Attending parties--especially an archenemy or ex-girlfriend's party--is a wide-open opportunity for evil acts that are limited only by the darkness of your soul and imagination. Having sex in the host's bed or using his toothbrush for an anal scrub are always good choices (taking pictures of the act and mailing them a few weeks later is even better). If the host has an old school answering machine, change the message to, "I'm sorry, I'm too busy licking balls right now to come to the phone." Or try the old "hide a pumpkin in the back of a closet" trick! It takes about a month for the pumpkin to fully rot, but when it does, it smells worse than a decaying corpse. As an impromptu move, pooping without flushing is always an irksome anonymous housewarming gift.
But perhaps the crowning delight for an evil partygoer is stealing photographs. At each party my anonymous friend "Dave" attends, he "borrows" one of the graduation, wedding, or special-moment photographs magneted to the refrigerator. He takes one of these cherished Kodak moments home to his own fridge, which, after two years of thievery, looks like a Japanese tourist's photo album. However, if photo-theft is your game, be careful who you invite into your home. They may be surprised to see their own memories posted on your fridge. "Dave" was recently busted when he hosted his own house party.
"Hey," an irate man asked while standing in Dave's kitchen, "how come you have a photo of my dog?"
Dave tried to pretend the dog was his cousin's golden retriever.
"Yeah?" the guy responded. "Well, what was your 'cousin's' dog doing in my backyard?"