take all the interior doors off and put them in the garage or back porch and hide the pins to the hinges through out the house, turn all the furniture upside down and place whatever was on time (ie flower, candle) back on them, take all the lightbulbs out and hide them or unscrew them just enough that they don't work (makes them think the breakers are blown). Kep it clean because you don't want to ruin the fun just want to pick on them.