Wanted something to keep everything I might need out of the house without making holes in my pocket, and still leaving room for a separate card wallet. Fold open is very helpful, both to find things and fit things in, and allows room for a thin 3500mAh power bank. If I'm nitpicking, could do with more rubberised zips, a small interior zip section, an external pouch (e.g. for tucking keys into) and the half-sewn pocket is weird (and easy to lose things under); velcro would have helped - but I've not found better competition at the size. I avoided the hook and loop version in case of pocket pick-up, but in retrospect it might have helped for connecting keys to the exterior.
My (highly over-packed) contents are:
Front zip: 2x compressed towels, microcord, USB adaptors (C<->micro B, A->C, C and B NetDot magnetic), emergency keys, USB stick, small strong magnets, S-biner micro carabiner.
Left half: disabled toilet key, mini sharpie, fibre pens (Schneider link-it black and red), NetDot 30cm gen10 magnetic cable, laser gun sight (in case cats get out), micro scissors.
Right half: 20cm USB C, tooth picks (under duress chopsticks), collapsing metal straw, tweezers, 2 tic-tacs in a pill case, 2+ days of medication (partly in the toothpick case), cable ties, Auskang 3500mAh power bank, £20 cash, alcohol wipes, plasters, Gorilla tape, self-amalgamating tape, TrueUtility ball point pen, 2x post-it notes, 3x write-in-the-rain paper, business card (mostly for id).
A key (in a rubber sheath to protect my pocket) and couple of LED torches (I often need to lend out, so a Tiny2 and a Frog) are on an external coily cable so I don't have to get this out of my pocket to open the door - but being my main house keys I'm reminded to bring it when I travel. This is all safe for airline carry-on, so I won't get separated from it. Total thickness is about 6cm, with room left in my pocket for an inhaler and credit card wallet.
I have a smaller/thinner option (zip pouch) for around the house where I have different needs.
Not that I travel all that much (without a bigger bag), but I'm confident of having things to hand on a plane or car without needing to rearrange things, and mostly I'm secure against forgetting something important.
Black X-Pac X50