The coins in a cash register are organized from highest to lowest*: quarters, then dimes, nickels, and finally pennies.
Using these four coins as place values, I have a new, albeit very limited, counting system. It has only 4 place values, obviously there can be a maximum of 4 in the pennies place value, 1 in the nickels, 2 in the dimes place, and 3 in the quarters place, and finally, the whole number must not add up to 100 more (in this case, you would give the customer a one dollar bill).
To make this more interesting, I occasionally substitute a single skipped place value with a decimal point. This means that 31 cents converts to 1.11. Of course, this is not very mathematically sound, but things work better this way.
The effect is to create another level of numbers above the monetary value of the coins returned, making transactions a little more interest.
Here are some of the most notable results:
41 cents -> 1111
84 cents -> 3.14
17 cents -> 0112 (the first four fibonacci numbers, but is arguable less cool because it break the skipped coins = decimal place rule)
I'm sure there are others, but those are the only ones that come to mind.
*Maybe some registers do this in reverse order, but I declare them non-canon.
In other news: you can tell while steaming milk when it is at around 140 degrees by the sound it makes.