one of the best ones i have done.
prepare before hand:
crunchy peanut butter mixed with enough marmite (veggiemite) to make it into a nice turd colour, if its a bit slimmy add a tiny bit of flour to dry it out
wrap in glad wrap stash in pocket.
go to a friends house who has a pet (dog/cat)
bail to toilet, remove said material from glad wrap roll into friends animals turd size.
strategically place fake rolled turd in the lounge or area where you think would be fun
make a big noise telling owners their pet has laid a cable in a couch/floor or where ever you planted fake turd
after a few moments of everyone going "yuck etc" offer to clean up, pick the fake turd up and make sure everyone can see it in your hand, proceed to smell it, compliment the pet owners on the type of food they feed their animal "smells like whiskers"
if you guessed wrong "which you should" say, "no im sure" take a test bite of said fake turd.
laugh inside while people start vomiting and all the chicks are fully groused out
best played at a party when everyone is maggot