A 7-in-1 multitool in credit card size (although it's rather thick...)
- 12mm, 10mm, 8mm, 6mm, and 3mm hex nut wrenches
- 6mm screwdriver bit holder
- Bottle opener
- Ruler (metric)
Tried to fit as many functional utilities as I could on something that could be made of plastic.
Ruler is metric; each tick is 0.5cm and each tick width is exactly 1mm.
Whistle is surprisingly super loud (it's on the opposite side of the ruler). Scraper is useful for your car during winter if you don't otherwise have another scraper for ice/snow buildup. Funnel is useful for precise liquid pours, and the screwdriver bit hole in the one corner is useful as a backup. (Alternatively, the bits also fit into the 6mm hex nut wrench)
Pretty useful little tool to carry around. And if you get it made of plastic, it won't set off metal detectors! (Great for overseas travel)
Any material--even PLA--should be fine, although I recommend printing in 100% infill for the best durability. No supports needed. You may need to scale up a bit for ABS prints & other materials that suffer shrinkage.