Stick the pack inside a plastic trash bag if you are worried about it getting wet. Take several, they weigh next to nothing and tear easily. Put anything you want to keep dry inside plastic bags inside the pack. You don't need fancy dry bags, the big zippered plastic food bags work pretty well and will last a while. Most packs are waterproof up to a point anyway, but things like underwear and socks you want to keep spares dry.

Not the original question but make sure whatever you are wearing will keep you warm if it gets wet, don't even take cotton underwear if the weather is going to be cool. I've got some wool sweaters that will keep me warm even if they are soaking wet and synthetics like Polarfleece wil do the same thing.

When I was hiking in NZ and rain was a common annoyance, I kept my pack in my tent, but I had room for it-another reason I think bivys are a bad idea for most people in most conditions.