I keep gloves, 20 psi tire gauge, and a couple of clean wipes on mine. The tools I keep on my service truck to prevent them from breaking the cab glass in rough fields.
This is sooooooo dependant on tractor use. I carry clampon cylinder stops, 3pt arm bushings, several hitch pins, extra set of 3pt clips, PTO shaft converter, 2 chainsaw chains, 1 chainsaw, 2 adjustable wrenches, fencing pliers, hammer, clevis, 3 log chains, extra 3pt top link, extra 3pt side link, bug spray, toilet paper, can coolie, cooler, prybar, tordon. I'm positive I missed something.....
LS P7030 CPS, 2016 Bobcat E42, Ferris IS3100z w/37 HP Kawasaki
You will make up your own must haves pretty quick. Whatever you find yourself driving back to the barn/shop/house for is a good start!
I don't carry a lot of tools. Invariably when something breaks/loosens etc, it's a trip back for the big tools anyways. Good maintenance lessens your need for tools while tractoring! Something to tighten up loose hose clamps can be handy and wire cutters.
Sunglasses, gloves, a few shop towels, grease, duct tape, a few bandages and cell phone about does it for me.