I've learned after getting to the range only to go
damn I forgot the . . .
Extra empty magazines
Ear and Eye Protection
Batteries for ear protection
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.
Water / Soda
Spare Change for vending machines
Cash for targets and the like (they hate it when you use your credit card for $2.50)