A socket set that has a 1/2" breaker bar and socket, and the sockets.. say.. 3/4 up to 1 1/8 at least.
A brass or copper 'maul'. Brass and copper are non sparking.. and also won't mar up cast iron or steel castings and parts as bad as a hardened steel 'framing' hammer.. or a sledge hammer.
A set of drifts or punches, including small ones for driving roll pins.
A good set of wrenches in large sizes as well. get the type you need for your tractor.. either sae or metric.
A propane plumbers torch is also great to have on hand.
A grease gun.. either hand type, pump.. or if you have rich tastes.. and electric or air driven one.
At least a 'pancake' or other small compressor at a minimum.. so you can air tires.. and use a blow nozzle to blow debri out of things.. or run that nifty air grease gun.. perhaps even a small 3/8 air tool.. etc.
tire pressure gauge
Any special torx or allen tools your tractor may need to do things like disassemble dash.. etc. set screws.. etc.
Other generic items like a white grease marker for marking leaks or areas of interest on a tire.. clean up rags, a good funnel that has a top and bottom screw on cover so it will stay clean.. along with at least 1 5g fuel can o f the appropriate color.
A pry bar of some sort... Tire spoons can be used.. though if you don't plan on breaking down tires.. and metal pipe will do, and it will also doube as a 'cheater' for that 1/2" breaker bar.
A bag of bunji cords.. and eventually.. I'll bet you will need some chain.. if not for trailer tie downs ( or ratchet binders ).. for other items.. like lifting with the fel..
A good mechaincal tractor jack. Even the 35$ cheapy TSC kind is fine.. and a good set of jack stands.. .. because you never get under a tractor that is supported by a jack, or concrete block.. only one on good cribbing.. or jack stands.
A handfull of lynch pins, roll pins, and hair pins to throw in a baggie int he tool box... brush has a tendency to snatch those pins off at bad times..
To this end.. a ball of mechanics wire or bailing wire and a set of pliers and an adjustable wrench int he tool box may be a life saver. A good size adjustable wrench can be used as a hammer or wrench.. in emergencies.. and the gas pliers are ok for cutting wire, and great for anything the wrench won't fit... A scredriver/nut driver with a few tps is also a good idea, as well as a first aid kit and a fire extinguisher somewhere on the tractor.
A cup holder, and a safe holder for a cellphone is also nice.
Perhaps a couple extra misc soft bolts int he tool box including nut and washer hardware, and at least one extra shear pin for the mower int he tool box. This saves trips back home/to the barn, etc.. to make minor repairs.. etc.
The list can virtually be endless.. however I listed most of the stuff I bought when i got my first tractor, and tried to list the stuff I carry on my tractors when I leave the barn.. etc.
Soundguy