REI Co-op Flash 18 Pack vs AER Go Pack

Unlike other reviewers, I find this pack to be in the small side. It's great for quick hikes to hold a small jacket and sandwich, but doesn't hold much more than that. I've been trying to make a crag-bag out of this, but I can't even fit my harness and shoes in here, let alone jacket, food, other gear, etc.

These are going to be more suitable for ‘true’ one bagging (ie when you travel without a checked bag). If you decide to opt against a checked bag, perhaps look at a packable day bag and one of your options above, or take a look at the Peak Design Travel Bag, as this is designed with photographers in mind, is effectively a big bucket of a bag, and can expand from 35-45l (although I think when expanded to the full 45l it is slightly too big for most airlines). For a daily carry, you ideally want to be looking at something 25l or less, even under 20l.