Over the years I've made several pocket gages that are handy to carry on shopping outings where one doesn't need a full complement of measuring equipment. The one on the bottom is a copy of a classic lumber gage. Dangled from the forefinger, one snaps it over the edge of a board and reads the thickness directly.
Upper right is a simple gage for surplus screw buying. It has tapped holes for all the common sizes that appear, unmarked, in some of the liquidators I frequent. It's about the size of a quarter so fits easily in my change pocket.
Upper left is a handy bolt sizer made by milling an N x N slot where N is every endmill size from 1/16 to 1/2. The more I carry it the more uses I find for it.
At the very top is a magnetic pick-up stick. It's there to remind you to carry one every time you shop Home Depot or Lowes type stores for "brass" hardware or "copper" wire. You'll be surprised at how many of the aforementioned stick to the magnet.
Resist the urge to glue a magnet to one of these tools lest it magnetize stuff in your pockets that shouldn't be magnetized. It may be OK but I would want to keep my smart car key away from magnets.