Like many folks I use a credit card sleeve as my wallet. Super slim, simple, cheap. One issue with that is when paying for stuff you can't see the face of the card, so there tends to be some rummaging for the right one.
Inspired by one of my cards being yellow, and therefore easy to select: enter permanent markers, and some nail lacquer.
Different cards get different patterns and colours, and once dry, a coat of clear nail varnish (don't skip this bit, I found even permanent marker rubbed off after a few weeks). Voila: easy to find cards - once you learn which pattern is which of course.