Performing something with a regular deck at short notice is always a challenge, especially if you have just shuffled or borrowed the cards!
In these situations, I like to go for something self working, and that way you can concentrate on putting the effect across and also just connecting and interacting with your spectators and having a good time.
Over the years, I've created and collected a pile of tricks that meet these conditions, but recently I was reminded of a Charles Jordan trick (with some Dai Vernon embellishments) that was a real fooler, and if you can borrow a deck of cards (doesn't have to be complete) and a pen, then you are good to go!
Start off by removing a Joker (or other prominent card) and writing a prediction on the back without letting anyone see. You write the numbers 5 and 6, but with space between the digits.
As you do this, have the spectator remove any 20 cards at random (I have them shuffle and then just count off the top 20).
When they have removed them, they shuffle the cards again and you drop the face up card onto their packet, and invite them to shuffle more.
When they are finished, have both the card that lies above the Joker and below the Joker remembered (if you have two spectators, they can remember one card each). Once this is done, have the cards cut a couple of times.
Take them back, and you now do a procedure called a Reverse Faro (or Anti-Faro) by up jogging the first card, down jogging the second, up jogging the third, down jogging the fourth, and etc all the way through. When you have done that, remove the up-jogged packet by pulling it free from the down-jogged one, and drop it on top.
The same result can be had by dealing the packet into two piles, and then dropping the left hand packet onto the right hand packet.
Do this twice, and then follow up with some cuts, and let both spectators cut again.
During this, the spectators have seen the reversed Joker, but that's fine, it adds to the conviction that the cards are being shuffled.
End the trick by cutting the reversed Joker to the top, then tabling it. Spread off (don't reverse the order) the top ten cards, and table them to your right. The other packet is set down to your left.
Hover both hands mysteriously over the packets, and 'divine' that one card is 5 cards from the top (the right hand one) and the other card is 6 cards down from the top of the left hand packet.
Let the spectators check this out and reveal that it's true...
Then reveal the kicker and turn over the Joker, which somehow revealed the locations of the cards BEFORE the trick even started! BOOF!