Some coin-operated
washing machines or
dryers, as well as pool tables or other similar arcade games (air hockey, etc) use a coin-op mechanism that consists of a sort of metal slide that you place your coins on and then push into the machine.
These devices are easily circumvented using one of those brown "Mr. Stir" plastic coffee stir-sticks (or equivalent).
This will be easiest if the machine only requires one coin, but works for any amount.
Simply insert the stir-stick into the slot and jimmy it about while pushing inwards on the metal tray. Eventually you will be able to trick the machine into thinking that there is a coin in there and the tray will slide all the way in.