Cooler for food/drinks
If none at the range, a lightweight folding chair, and maybe even a small folding table (for reloading mags, working on gun, eating . . . or use your cooler, or small tarp).
Microfiber cloth for cleaning glasses, optics, etc.
Second the FroggToggs: cheap, work well, and good to store in your vehicle afterward
Hand towel, for rain, sweat, etc.
Spare footwear & socks, just in case rain stops.
Backup weapon, or at least spare parts (springs, striker/firing pin)
If you're not used to high round count days, maybe some moleskin to put on any developing hot spots (or gloves, or bandaid/flextape)
Don't hesitate to ask questions. Explaining and understanding don't work the same for everybody. If something isn't clear, ask for clarification.
Enjoy!!