Borrowed deck, no prep? I like Jim Steinmeyer's 9 Card Problem. You do not even have to touch the deck. Easy to do, and very good reactions.

Thought of card at any number ...
the classic 29 card effect.

WulfieSmith

Emotional Reaction.

Jon Armstrong's Out of This Blah Blah Blah and also Red Herring by Benjamin Earl

3 Possibilities - DaOrtiz.

I am definitely in the Gemini Twins camp. If I only do one impromptu card trick, that's my go-to.

As others have mentioned, open to so many possible presentations. In fact, I did one of my favorite versions for my wife's physical trainers on Valentine's Day, (which I call Two Brothers) using the red Jacks as locators for the red Queens,

The idea is that the brothers are similar, but have somewhat different life goals (financial vs. emotional). The dealing through the cards represents their progress through life, and in the end they find their perfect mates.

(I'm sure it has been done before. If anyone knows of previous similar presentations, I would love to know.)

I can do the setup right in front of the spectators, almost everything can happen in the spectator's hands, and it always seems to get a good reaction.

Take a look at Matt McGurk's YouTube channel. He demos and explains many great and easy self-working card effects.