My favorite coin trick is called scotch and soda. It's so simple, I taught it to a five year old in about 30 seconds, and he spent the rest of the Christmas party blowing adults' minds.
MAGIC SECRET SPOILER AHEAD
The trick coin is two pieces. The obverse is a hollowed obverse of a half dollar (I actually discovered one of these in a roll), similar to this coin, but the hollowing goes almost to the rim. This is the soda.
The reverse is a plug made from a half dollar reverse with the rim sanded down, stuck to a Mexican 20 centavos, which is just sightly smaller than a half. This is the scotch.
You put any coin smaller than the 20c in the soda (a Brass Buck will work, an Israel coin with scalloped edges or a square coin will blow minds), and stack them on top of the scotch. You can now slide the soda around on top of the scotch, showing that it's "solid". Then you take the scotch and move it around on top of the soda. Finally, stop the scotch exactly on top of the soda, and point out that it's a little smaller. Keep in mind that while I am doing the patter, the 5 year old is doing all the manipulation.
Now, you ask the victim to put out their hand, palm down. Very obviously, take the two shown coins and put them under the hand. As you do, slip the hidden coin out and squeeze the scotch and soda together. Ask if they can feel two coins, and confirm that one is smaller than the other. The palm of the hand isn't very sensitive, but they'll be able to tell.
Now have them put both hands behind their back and split the coins between them.
Ask if they can tell which one is bigger, which they easily do. Have them bring that hand forward and open it, and sure enough, it's the half, which you can show both sides before pocketing.
So now in the other hand, you have the? And they answer "the brown coin" or "the Mexican coin". Ask them to open their hand, and there's a scalloped coin from Israel. Since they lost your 20 centavo, you might as well let them keep the coin in their hand (a neat way to pass out coins to kids).
You can repeat the trick, close up, multiple times, using
V nickels, Buffalos, foreign coins,
Mercury dimes, SBAs,
IHC, whatever. It just gets more confusing as you go.