I run on both sides of the fence about my haversack. When campaigning, it's full of food only, the purpose for which real soldiers carried them.
At mainstream events, it holds my hard case with my modern glasses inside; a little plastic bottle of eye drops, maybe my turned-off cell phone, and not much else. The haversack is mostly for looks at mainstream events, with just a couple of hours for the battle away from camp. I never put plastic water bottles in there, I use my canteen. I may put in an extra one or two arsenal packs of cartridges-to give to others usually-and a tin of priming caps.
I never put my wallet or car keys in my haversack. They always remain in my trouser pocket. If I'm taken off in an ambulance, who knows if anyone would send my haversack with me. And I'm so paranoid about losing keys that I often use a safety pin to pin the keyring in my pocket.
I always carry a big cotton checked handkerchief for all sorts of uses, and that always is in a trouser pocket, not the haversack.