My AMG (ass magic bag.) Just having it lets me carry more stuff, able to throw random things we need into it, and is a convenient way to carry my spell book and magic missiles.
Paintball Speed loader bag it lets me carry two bottles of water on me at all times
Nail clippers. This might sound really girly, but it's super annoying when you rip part of your nail off while fighting and it's jagged and uneven the rest of the weekend.
deck of cards
Clean socks and sneakers, left in the car to avoid rain. I hate driving home with wet feet.
Towels. Every time I have them, I don't need them, but every time I don't have them the heavens open and god cries on me.