A fun effect to perform, having a two surprises, the second being a kicker ending!
You show a deck of cards consisting of cards from many different decks having several different back designs.
You remove four cards face down, explaining that it will be a Poker hand as soon as one more card is added to complete it. Your spectator chooses any card they desire, and you show that the card they selected (the Ace of Hearts) makes the hand a Royal Flush! They assume that all of the cards must be "the same." You show that indeed they are, but they're not all comprised of the Ace of Hearts -- all of the cards are, in fact, BLANK!