I use a messenger bag, a lot of people don't like the bite in the shoulder but I wear it everyday to campus so I'm used to it, and it's served me well at pax and sdcc.
For me the reality is that your not going to have a lot of weight from just one day of swag, you'll get a good amount of swag but tshirt aren't that heavy, the plus of a messenger bag is that it's easy to a maneuver to the front if your in a crowd and worried someone will get to it or you need to get things out. Another plus is that I heat up easily and the messenger bag keeps my back from getting to swampy.
I'm local to the convention center here, so I don't bring a backpack for anything, just messagner bags for my daily essentials and carrying small swag purchases.
As such, the biggest feature in my away convention backpack was a camp stool.
It was $35, small side/rear pockets for snacks, two drink holders, and a main pocket for swag or a sweatshirt. I leave the stool strapped to it, and sit the whole thing together.
I've used it at least 3 times, and get nothing but adulation from fellow attendees. Nice to be able to offer it to tired friends too.