I travel with two bags. My main bag is the MEI Voyager, and naturally contains all my main gear. Clothes, toiletries, laptop, extra batteries, etc.
My second bag is a Think Tank Speed Demon 2.0, and it holds my camera, lenses, etc, and when I'm traveling, a water bottle, perhaps spare change, maps, etc. Anything I might want quickly.
The Think Tank bag is marketed specifically as a camera bag, and I use it for that, even when I'm not traveling. It's ideal for travel photography, because it wraps around my waist, so my camera (and extra lenses, external flash, or whatever other accessories) are always easily accessible--even when I'm wearing my travel pack on my back! Ken Rockwell first turned me on to the Speed Demon 2.0 pack with his review. (While I disagree with a large portion of what Ken Rockwell says, his reviews on usability are often valuable, IMHO).
I spent a month backpacking around Mexico with this setup, including several long days carrying both bags with me all day long. (Of course, when possible, I'd leave as much stuff as I could in a hotel room, but that's not always possible.)
For my next backpacking trip, I intend to add a third back-mounted daypack, that I will stuff inside my travel pack. I'll keep my laptop, swim suit, and other smaller items in there, when it's possible to leave the bulk of my belongings in a hotel room, but my waist pack isn't big enough.
If you don't carry many camera lenses, a similar waist/fanny pack would probably work well for you, and it doesn't reduce mobility or cause you to overheat as a front-worn backpack would. If you do use many camera lenses, I still suggest this setup, but perhaps with a larger fanny pack than the one I use.
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