General:
* Ruse: A clever trick used to deceive someone.
* Stratagem: A plan or scheme used to outwit an opponent.
* Maneuver: A skillful or cunning movement or action.
* Ploy: A tactic or maneuver used to achieve a desired outcome.
* Trick: A deceptive or cunning act.
* Deception: The act of misleading or deceiving someone.
More specific:
* Double-cross: A betrayal or a deceptive maneuver.
* Bait and switch: A deceptive sales tactic where a customer is lured in with a low price and then persuaded to buy a more expensive product.
* Con: A plan or scheme intended to deceive or cheat someone.
* Charade: A deceptive or misleading appearance.
* Sham: Something that is not genuine or authentic.
* Hoax: A deliberate deception intended to trick someone.
Informal:
* Stinker: A particularly clever or effective trick.
* Gotcha: A surprise trick or maneuver.
* Shenanigans: Mischievous or dishonest behavior.
The best word to use will depend on the context and the specific type of sly trick you are referring to.