For everyday, I use a messenger bag by Timbuk2.
On rare occasions, such as when I’ve worn a sarong out to a club, I carried a small purse by dickies.
Keys with thumbdrive keychain in right pants pocket, any secondary thing I might be carrying in the left. Cell phone tucked snugly in one of those mini-pockets most jeans have on the right side (so I can keep it on vibrate and always feel it ringing). Wallet in the back right pocket. Sunglasses either on my face or hanging on my collar.
If I’ve got a lot to carry or I’m out to buy something which I’d have to carry back, I’ll bring my Israeli paratrooper messenger bag thing.
I have a bag like this which is great for day trips, or evenings out, but not so much day-to-day. The value of say, a pocket knife, is directly proportional to its proximity. If you need a knife it’s not much help knowing you have one in a bag hanging on the back of the chair in your office.
One of Middlebro’s nicknames is Tools Man or Pockets Man, because he always carries so much stuff; he bought his first Men’s Handbag about 12 years ago, when they were still a novelty, because he’d reached the point where his cargo pants needed extra pockets. I helped Lilbro shop for one a few months back; they’re perfectly mainstream nowadays.
These bags are about 8"x6"x2", hung from a shoulderstrap. They’re a lot smaller than school backpacks or laptop bags. I think the existence of these have helped a lot in making the smaller bags commonplace. Back when Middlebro got one, he had to just select a small woman’s handbag that wasn’t too… femaleish; now they’ve got decorations and colors that are absolutely man-geared.