I have a Game Plan fanny pack with shoulder straps that is pretty much loaded with everything I have needed over 40+ years of bowhunting.....it is pretty stout. I also have a Summit Viper SI climber with a Summit bag to hold my RC harness/Qsafe and such, plus a few key items such as a headlamp, TP, knife and field dressing kit. If I shoot a deer, I figure it is going to take me two trips back and forth....one returning with my climber and the other with the animal. This winter I read a lot of AT threads on the "bow and go" system and came up with a pretty workable system....a Millennium M7 microlite stand, 3 Hawk Helium rope mod sticks and a small kids camo backpack to contain the same necessary stuff as my Summit climber. I weighed my Millennium setup and it comes out about 22# total. So wearing my RC harness and carrying my bow/quiver, I have a pretty manageable setup for an impromptu hunt from the air.
Frankly, you will determine what is "necessary" by how much weight you can comfortably haul. If it is too heavy, then go through everything and follow Hawkdriver's sage advice.....if you don't need it that day, leave it in the vehicle. There are times I wish I had my Hooyman telescoping saw for a particular annoying branch, but I don't haul that saw in and out every time for those instances.