Put a radio control car out of control, then when it smashes into Santa (if it does) he'll drop all the presents and you'll know what they're shapes are because you won't have to dig deep into the stocking.
Basically try and think of a joke to play on Father Christmas.
Here are five examples:
Instead of putting out mince pies and sherry, leave him a glass of water, a piece of bread and a note saying you heard he was on a diet.
Leave him a note saying you've gone away and could he let the...