I purchased a range bag yesterday from Primary Arms. So now I'm working on a list of items to stock it with. Please list a few items you feel are "must have" in a range bag. No need to mention ammo
Gun range keys (you don't want to drive all the way out there and not be able to get in. Don't ask me how I know this)

Internal lock keys for any firearm you take that has the unfortunate device
Hearing and eye protection for two (you may take a friend)
Break Free CLP
Cleaning kit with rod, jag, patches, etc.
Good set of hollow-ground gunsmithing screwdrivers (not hardware store)
Semi Auto mag loader like UPLULA
Toilet paper (better to have and not need), hand sanitizer
Shop rag or old t-shirt
3/32 inch punch or Glock disassembly tool
1911 bushing wrench
Quality set of hex keys and torx keys
staple gun and extra staples
targets and pasters to cover holes (not in range bag, but next to it, so you don't forget them. Don't ask me how I know this)

Zip lock bag to put your carry ammo in while you are shooting your carry pistol
It is best to have duplicates of all of this stuff for your range bag and for your house so your range bag stays packed all the time and all you have to do is grab guns and ammo and go.
In addition to the contents of the range bag, you should have these items handy:
weapons
practice ammo
extra mags or speed loaders
holsters
magazine or speed loader pouch
any weapon-specific tools needed for a particular firearm
You will add stuff as your experience dictates.