Okay. I know this will sound funny, but I used the Patagonia Fly Fishing Sling bag for my recent trip to Asia. It cannot be used for business (no sling can), but for off hours/vacation mode it certainly can.
I prefer messenger-style bags for crowds, because it is easier to navigate through the throngs. Yes, I have TB's Ego, Super Ego, Pilot, and co-pilot; I just wanted something smaller, easier to tote through crowds, and try something different. Earlier, I had tried Pacsafe's sling bag. I liked it, but it did not address all my EDC needs.
The Patagonia sling held: my tablet, long sleeve shirt, snacks, water, rolled up slick (for the rain), point and shoot camera, charger, spare battery, water bottle, emergency toiletry kit, and still had room (not much though).
It is waterproof, which came in handy several times and smaller than a tactical sling bag, which I was thinking of using until I came across fishing sling bags (there are countless others, if this one does not suit your needs). As a side benefit, it was an ice breaker with many a local. It will never win any beauty contests, but it is fully functional and I highly recommend it.