A tiny keychain tape measure. It's one of those things you have no idea how much you'll use until you have one on you all the time.
Looking at something online? How big exactly is it? It says 16.5" but can you picture that in your head somewhat accurately? It's just easier to pull out your tape measure and see.
I've also started carrying around a small flashlight everywhere I go. I have an Imalent LD70 Mini. It's small enough to fit in my change pocket and it'll put out a blinding ~4000 lumens on turbo (for about a minute before it gets too hot). I do admit that it's not the best flashlight ever. It has a ridiculous magnetic proprietary charger that will short on whatever if you leave it plugged in but it does what I need it to do.
I have to admit I don't get the flashlight, unless you don't have a phone. Tape measure is funny though. I have to admit I've been in that position many times about ordering online, but usually I wait till I'm home to measure 🤣
Don't discount the flashlight so quickly. I avoid an excessive amount of shit in my pockets and it's always the same. Money clip, knife, truck key, pen, phone and my flashlight. The Flashlight is fairly small, takes a AAA and is slightly bigger than an AA battery with a pocket clip. It gets used multiple times a day. Makes small print in dimly lit areas easier to read also. Honestly for some reason I despise using my phone as a flashlight:)
My tiny flashlight will light up my whole yard and then some like it's day. I do admit that's a little bit excessive but it's great for when I hear something out there at night. I don't exactly live in the best neighborhood.. Makes clearing the property a hell of a lot easier and when that things pointed right at you, you can't see shit.
Even on the lower settings though it's far better and faster than using your phone. Also I guess it's a little bit more specific to my needs because I am a machinist, it's a lot easier for me to just pull out a flashlight than to strain my eyes trying to see something.