I struggled with the same question. The gearhead in me wanted the coolest, most functional pack available. I ended up getting 2 packs on clearance at Target. I focused on something comfortable and w/ large capacity. Got 2 Jansport day packs for under $40 each. The packs have 2 large compartments and 2 small front pockets. If we have to hoof it not all of the contents will remain in the bag. Sheath knife, psk, nalgene bottle, and a few others will be removed for belt carry. Something to consider when seeing how much the pack weighs.

I have not pulled them out of the car since I set them up (a year or so ago), except to check the contents. They do get pretty abused, sliding around the trunk and getting other items piled (and sometimes spilled) on them. May be something to think about before investing in something cool but that won't see a lot of real use.

I like to line the big pockets with a trash bag, then put stuff inside that. Using a trash bag on the outside while wearing tends to be cumbersome and prone to ripping. I give the cheap packs a coat of spray-on waterproofing on the outside, just to try to keep them from soaking up too much water.