Here is a riff on this idea that might entertain you -- it sure entertains the kids! I've been performing this one whenever I've had the chance lately, and it goes down very well (pun intended).

Background:

- In Canada, we have "loonies" - gold-colored coins slightly larger than a US quarter/the same size as a UK 2p.
- Our candy makers also make "chocolate loonies" -- gold-foil-wrapped chocolate coins that happen to be the identical size to a "silver maple leaf" -- a large silver-dollar-size coin.
- In 2004, the Canadian mint made a set of variable-size "silver maple leaves", one of which (a $3 coin!) is the identical size to a "loonie"

*whew*

Now, I borrow a loonie, and introduce the silver-dollar-size "maple leaf". I switch the silver coin for the equivalent-sized chocolate one, which the spectator holds in a fist. Taking the loonie, I switch it for the $3 coin, and hold it in my fist. Retrieving my magic wand ;) I claim that I will make the two coins magically jump back to their rightful owner. After some by-play, we open our hands to show that my volunteer is actually holding the loonie (albeit a chocolate one), and I am actually holding a (miniature) silver coin. Of course, I offer to buy the chocolate coin back for a dollar...

Thanks Francis; thanks Pete!