These are three of my "go to" self working tricks. Tutorials are widely available on YouTube. I will provide links after briefly describing the trick in case you already know it.
1. 21 Cards
I saw this on Danocracy's channel on YouTube. The effect is simple. The spectator is shown 21 cards and asked to think of one. The cards are dealt into three piles, three times. The fourth time the magician deals the cards he stops midway and reveals the spectators card. I like it because the spectator never actually has to touch the cards, merely think of one.
It will be the first trick presented in this video:
2. Four Aces
Another widely known and easy to perform trick. After a false cut and a riffle shuffle the magician begins dealing cards face down into a pile in front of the spectator until the spectator tells them to stop. The magician then deals out the cards (or has the spectator deal out the cards) into four equal piles from left to right in sequence. Naturally when all the cards are dealt and the top card is flipped over it is an ace. Some variations have the spectator cut the cards into piles. Plenty of versions on YouTube but usually a winner and requires minimal set up and can be reset quickly in the bathroom.
3. Kings and Queens
The magician pulls out the kings and queens and assembles two piles in chase order, one for the kings and one for the queens. The magician spreads the cards out in front of the spectator and asks them to think of a card. The cards are then gathered up and one set is jogged up while the other is concealed. The magician asks if the spectator sees his card (this works best when the spectator writes down their card to avoid liars). The magician gathers the cards and asks again and then a third time. At this point the magician deals out the cards in two piles and flips a packet over revealing the color, deals out the cards revealing the suit, deals out the cards revealing the value and finally flips over the spectator's card. This one usually blows people's mind. Minimal set up and instant reset.
YouTube Performance and Tutorial: