There are some things that you carry on your person at all times. Most of those can be found in your wallet, since carrying an ID and some form of money are usually a good idea. Since you’re going to have those objects with you, why not take the extra step and make them even more useful? For instance, you could† turn your ID or customer loyalty card (I recommend the latter) into a ruler.
These Cardsticks are the sort of thing that seem rather silly at first glance. However, when you† think about it, they are rather ingenious. I can’t count the number of times I’ve needed a short ruler and instead had to simply guess, because I had no such thing. These stickers don’t harm your cards, and provide three inches of measuring space in both inches and centimeters. Just $4 will get you three stickers, one each in white, yellow and clear.