If you're trekking, then a backpack with some extra space for a waterproof and water bottle is very handy.
The Lowepro AW ones are pretty good, but get the ones that have a separate camera gear compartment vs. other stuff. You don't want your water bottle or lunch leaking into your gear.
I have also used a waterproof top-loading daypack (actually an
Aarn pack made in New Zealand), for short trips into Europe. It's carry on baggage sized, and I can fit the insert from my hadley small into it, making a secure non-obvious camera backpack. Actually I generally leave the backpack in the hotel and walk around with the hadley once I've arrived at my destination. I have a set of the front pockets for the Aarn pack, and intend to modify them to use as lens bags.
The problem with backpacks as shooting bags, even if you're trekking, is that you have to remove them to get at your other lenses or to put your camera away. I also use an
Ortlieb waterproof camera bag which you can sling off the front of your backpack straps. It can probably hold 4-5 lenses and a body with lens mounted, and is submersibly waterproof when sealed up. I used it with the accessory harness to take a M7 and 4 lenses sea kayaking in New Zealand a few years ago. The supplied dividers for the Ortlieb bag are not M friendly, but you can use the ones from a cheap Lowepro bag as replacements.