I usually put a pack on my dog when we go out hiking (this one
, from Ruff wear). I usually just make her carry her treats and her collapsible water bowl in one pocket and a bunch of light rocks in the other, to balance her out. I try not to put too much stuff in the pockets as she's getting fairly old (and kind of lazy) and I don't want to weigh her down too much.
She doesn't seem to mind the pack at all and I have to say that she seems to like having a "job" when we're out on the trails.