You fan out a pack of cards and show the front and back.
52 cards, in no particular order, a good selection for the audience to choose from.
From the face-down deck, a member of the audience draws a card of his choice (freely, no force exerted), which is then put back into the pack. You really have no idea what card has been chosen.
Then you announce something of a sensation: The card, although nobody knows where exactly it is, will move up from the middle to the top of the pack when you snap your fingers!
You make a magic gesture and take the top card off the pack and show it to the audience - how embarrassing, it is the wrong card.
You explain that this is an extremely difficult trick, because the spectator's card is in the middle of the pack, it is locked in place by the cards around it - as good as being in a safe.
While saying this, you take a rotary knob with a scale ring out of your pocket and put it on top of the pack. Now ask some members of the audience to name numbers from one to ten. The number combination is then dialed by turning the knob. Then the pack of cards opens, like the thick door of a safe and there it is, of course, the spectator's card.
This can be repeated immediately, no in-between preparation required, therefore suitable for "walk around" and "table hopping".
Really easy to present - unpack, read explanation, and enjoy a successful presentation.