In addition to a real fire extinguisher I keep a spray bottle full of water handy. You can knock down a rag or random fire easily before it gets to needing a something real.
Silver sharpies are great for marks that can be read with goggles on. Fire blankets are handy, and a couple pairs of gloves of varying thickness and length. Wire wheels, flap discs, some acetone and rag for grease cleaning. Backup mask for helper or in case the battery dies. Roll of flux core is great if you only have 1 tank. Spare regulator if its cheap (if you have several welders this is easy). If you have a stick welder I would always keep some 7018 and 6011 on hand for when the mig can't cut it.
I hit up dollar general every now and then. The cheap Pam is great for welding spray, the DG brand oven cleaner is great for degreasing/paint removal from engines, transmissions, rear ends, etc.