I've learned after getting to the range only to go
damn I forgot the . . .
Range Bag:
Extra empty magazines
Speed loader
Ear and Eye Protection
Batteries for ear protection
Gun Oil
Bolt Grease
Bore Snake
Clean rag for the above
Small slotted screw driver
Hex wrenches for scopes and red dots
Small bottle of glass cleaner for the above
Padded bath mat to rest gun on (learned this from a guy at the range) They are usually the perfect size 18" by 24" or so.
Extras:
Water / Soda
Energy bars
Hat
Sunscreen
Spare Change for vending machines
Cash for targets and the like (they hate it when you use your credit card for $2.50)