Eye Protection and a spare set
Spare batteries
Spare chamber flags
Bottle of Water
I’d recommend at the bare minimum at least keeping a spare pair of eye protection and ear protection as well as a blowout/first aid kit as part of your essential items. Those three things are paramount to having a safe day at the range and you’ll find that you make a lot more friends when you have a spare set of safety equipment for them to borrow when they’ve forgotten theirs.
All in all, range bags are great and they are a core part of the shooting experience so choose wisely!