Originally Posted by Dan

Water in the oil, oil in the water - block the radiator and disconnect the cooling fan to force an overheat and blow the head gasket.

Sand in the engine?

Tea bags in the fuel tank will dissolve and cause multiple blocked fuel filters (so I'm told!).

Have an 'accident' after a curry, on the front seats and floor.

Get a child to be sick in the car, because it will stink for months.

Spill a pint of milk under the floor mats just before you hand the car back.

'Fray' the timing belt with a Dremel.

'Fray' the auxiliary drive belt with a Dremel.

Remove hub bearing nuts, pack bearings with sand, refit nuts.

Buy a couple of mice and leave them to breed in the car.

Carborundum grinding paste on the windscreen wipers.

'Accidentally' drive over a lot of large nails just before handing the car back.

Post the number plates all over the internet and encourage cloning to introduce administrative hassle.

Report the car as stolen after handing it back.

There's plenty more, but that's a start.