I do a lot of travel/event photography and I've found that a two-layer approach works well for me. I carry a Tamrac Adventure 9 backpack with most of my gear (laptop, batteries, cards, lenses, tripod, back-up camera, snack, H2O, etc.); I also have neoprene covers for the camera, one lens, and flash. That way I can have the main camera out all the time (with a comfy strap) nice and safe (from water, rocks, sticky fingers) and still have all the necessary stuff handy.
The backpack: tamrac.com/AdventureBacpacks.htm
The camera cover: optechusa.com/product/detail/?PRODUCT_ID=32&PRODUCT_SUB_ID=&CATEGORY_ID=5
HTH.
---Ritch