While browsing the big web, I saw some tiny Granny squares put together to a keychain coin holder somewhere ... There was just a picture, no sale, no description - and I just cannot find it anymore.
But as I thought it to be a great idea, I've tried to crochet something like this by heart - especially as you are always in need for a coin for the shopping carts, at least here in Austria.
It's basically made out of two tiny, simple Granny squares, crocheted together with a loop for your key ring.
I used a crochet hook 2,5 mm and a bit of thin sock yarn, but a strand of embroidery yarn would do as well, I think.
The important thing is just about to cover the coin required and then crochet the two parts together on three sides.
(If your little bag should turn out to be a bit too big to hold the coin, you could always diminish it at the opening with a few stitches.)
Another thing which I wanted to try out for so long now, are these recycling purses - they are quite handy as a beach money bag - or holding a travel sewing kit - or tampons, lipstick, a small mirror ...
At least I know where I found this tutorial, it's from the Austrian recycling-artist Regina Lustig's homepage - it's a free pdf-download - in German, but the pictures are so clear, you'll understand it anyway.
And while we're talking recycling, this is the link to morsbags, where you'll find a free DIY-tutorial to easily sew the plastic-avoiding bag yourself!
(German summary: Eine kleine Münztasche aus einem Rest Sockenwolle gehäkelt, ein Geldbörsel aus wiederverwerteten Getränkekartons - und der Link zum Selbernähen von einer Stofftasche - voilà!)