Bill Duncan points to a wonderful effect, the card in matchbox by John Carney. It's easy, but you'd be surprised at how hard that effect is for the average layperson to perform. I'd stick to really simple tricks, bar bets and gags. Here's some ideas.
1. Secretly reverse the bottom card of the deck. Have a card selected. While they're signing the card, secretly turn the deck over. The card is replaced, actually reversed in the deck. Put deck behind back and fix the bottom card, saying you're going to remove a card a reverse it in the deck. Then reveal reversed card is selection.
2. From Encyclopedia of Impromtu Magic, by Martin Gardner. Fingerplam a book of matches with all the matches torn out in your left hand. In your right pocket are the loose matches. Pick up a regular matchbook with all the matches still inside with your right hand and say you're going to "make the matches disappear". Do a really bad fake transfer. Openly put your right hand in your pocket, dump the regular matchbook and pick up the loose matches. Look at your left hand and proclaim the matches are gone. When the spectator says you palmed them and put them in your pocket, open the left hand and then open the matchbook showing the "matches" have disappeared. Then dump the loose matches on the bar for a kicker.
3. Teach them how to use a locator, like the bottom card or top card of the deck. Show them how you can build a simple location into a great mentalism effect.
4. Teach them a bill tear. You know, a quick one they can use when they pick up a tip before they drop it in the register.
5. Teach them how to vanish a cigarette in a thumb tip.
6. Teach them the Princess Card Trick.
7. Also from Encyclopedia of Impromtu Magic, teach them the trick in which two matches penetrate each other. You know the bit, where you push the matchhead into the fleshy part of your finger.