Two liters of water, a wallet, some snack, and a spare wheel.
It should not strangle me, bruise me or generally get in the way.
The one thing I do not like is sweaty clothing on the back. Another problem is the contents of a standard backpack that keep prodding my back.
It should not be hanging off a belt around my waist (been there before).
And I don't want a backpack with a water bag, I'd need to buy a new clean bladder every other day (or keep flushing it with some chemicals). I don't have a problem to buy a PET bottle for the drink every other day (until one day the EU bans them).
My result follows: