Knife and/or multitool (
duh!)

Ammo
Targets
Stapler (you will be the most popular man at the range:
Guaranteed)
Ear defenders & plugs
Cleaning & stripping kit (rod, patches, solvents, lubes, Boresnake etc.)
First Field Dressings & basic first aid kit
Front & rear (sand)bags for rifle
Legal paperwork (here in the UK I carry my Firearm Certificate)
Spotting 'scope or binoculars
Tripod for 'scope
Printouts of ballistic data
Manuals for weapons, sights, Mil-Dots etc.
Snacks
Water
Shooting mat (if required)
Breech flag (to show weapon is clear)
Rifle vice (MTM) for cleaning, sight adjustment etc.
Chronograph?
General repair kit of cord, duct tape, zip ties, safety pins etc.
Weatherproof clothing, e.g Gore-Tex, fleece (ranges are
coooold...)
Whistle to call an emergency halt to shooting
Personal ID in case of emergency
Keys for gun locks, car, home
Notebook & pen
Spares of much of the above, as your dim pals will forget something...
maximus otter