Borrow a medium-sized coin from a spectator. Ideally, you should use a quarter, but a nickel would also prove suitable. On one side, draw a semi-circle to represent a fishbowl. On the other side draw a very simple fish. Tell the person you'll attempt to get the fish into the bowl.
Place the coin on a flat surface and spin it as fast as you can. The coin flips between the image of the fish and the image of the bowl so quickly it creates the optical illusion that the fish is actually inside the bowl.
To make the effect more impressive, you can have an identical coin hidden in your palm on which you've previously drawn a fish inside a bowl. After spinning the coin once or twice, feign disappointment. Tell your one-person audience you can do better, then switch her coin for yours. After the coin stops spinning, claim that the fish must have gotten fed up with being spun around and jumped into the bowl.
This is a very simple trick that requires minimal sleight-of-hand skill. Prior to your performance, take two quarters and glue them together with a glue stick. Refrain from using superglue or any other strong adhesive.
Ask a spectator for a quarter, then quickly swap it for your two quarters. Place the glued-together quarters in the palm of your hand and quickly clap your hands together to separate them. Voila! One coin becomes two. This effect is best performed quickly and nonchalantly as filler between longer, more complex tricks.
Ask a spectator to take a quarter and, holding his hand palm up, balance it on his outstretched fingers. Since this is not really possible, he will inevitably fail. Ask if you can borrow the quarter and try it yourself. Holding your hand flat, take the coin and place it on its edge on the gap between your index and middle fingers. Prior to the trick, place a small, thin pin between those two fingers. While positioning the coin, manipulate the pin into an upright position using your ring finger. The coin will then rest against the pin, allowing you to keep it balanced.
Place 10 quarters in a hat and ask a spectator to pick one. Hand him a pen and ask him to mark the coin with a cross over the face. Ask the spectator to pass the coin around to everyone else for examination. While this is happening, take out the other coins and spread them face down on the table. Once everybody has seen the coin, turn around and ask your original spectator to place the coin face down among the others.
Turn around and touch each of the coins. The coin that has been passed around and handled by all the spectators should prove noticeably warmer than the others. You can then confidently turn that coin over to reveal the spectator's mark.