Turn off all alarms.
Delete all recorded programmes and delete scheduled recordings
Disconnect all power leads and cables from everything in the house. Jumble them all up (making knots if possible) remove any/all identifying labels and dump them in a heap on the floor.
Turn all the books on the bookshelves around so that you can't see the spines at all.
Mix gravy granules into the instant coffee.
Swap all tea/coffee in the house for decaf.
Put washing up liquid into the dishwasher rinse aid reservoir.
Leave the freezer door slightly open.
Hide odd, dirty socks all over the house, on windowsills down the die of the sofa, under cushions behind the booms on the bookshelf, under tables, in drawers, everywhere.
Remove the last page of every book in the house.
Get out all their board games, mix up all the tokens/cards/money/counters and randomly shove a handful back into each box.
Cut the plugs off all phone charger leads.
credit for inspiration for many of these must go to my DS