Use of money is a key math skill for life. It's also a good way to learn about addition, subtraction, and fractions. This Thing has five coins in the 1,5,10,25, and 50 denominations so you can print any combination you want.
The coins are double sided with the value on both sides so they are actually a little easier to use than real coins. They are approximately the same size as the corresponding U.S. coin.
Okay maybe they aren't entirely free. The 1 cent coin uses about 0.3 grams of filament which at $30 per kg spool, that works out to about 1 cent of plastic. The quarters use about 0.5 grams of filament for a costs of about 1.5 cents per coin.
Yes the title is intentionally based on the Patti Smith song. If you realized this go ahead and print yourself a few dollars to celebrate.
Put what ever set of coins you want on your printer. I printed with a 0.4 mm tip and 0.3 mm layer height and it worked fine. You shouldn't need a fan, but it might help with the overhang. You can see how mine came out in the photo (I didn't use a fan). Use grey/silver and brown/copper if you want to make them look more like real coins (I didn't have any brown or copper so I used yellow).
They are thin and simple so they print fast. I've even included the embarrassingly simple OpenScad file in case you want to change the coins.