I have a 5-ton and 10-ton porta-power that get alot of use working on farm equipment.
Build or buy an assortment of JIC plugs and caps, and pipe plugs and caps for hydraulic and sprayer repairs. I cap the line and fitting when I remove hoses, pumps, or cylinders for repair to avoid the oil leaks where you don't want them. Check with mechanic friends or your local parts house to find them, or get a kit set from e-bay.
The assortments from Harbor Freight are ok for most jobs, you will want hair-pins, star washers, o-rings, and roll-pins.
Menards has a big kit of standard nuts, bolts, lock, and flat washers that is ok for the basics.
Napa has a good metric assortment kit with nuts, washers, and short bolts.
Get a set of grease zerts in an assortment and remember to get a metric assortment too. I got a couple plastic parts boxes at Sears and filled them with the Deere metric zerts and their respective part numbers to re-stock it.
You will want an assortment of glass fuses, blade fuses, mini blade fuses, and possibly relays.
A Thread Cleaner set is always handy.
Invest in a car charger for your cordless tool batteries. I have mostly Dewalt 18volt tools and the charger in my truck is great for charging batteries when taking unpowered grain bins apart and the days without power in the winter.
Get a nylon seat cover from Snap-On for your truck, and or clean fuzzy tractor seats.
A big magnut with a release is great for finding nuts and springs in grass, corn, mud.
A good flashlight, and a snake-light or magnetic light.
A set of hose pinchers (Sears or any of the trucks have a set of plastic plier style ones that work well, you will need a lteast 2 of all 3 sizes.
A good funnel with a removable flex tube.
You will always want something from the shop, but with time you will become creative and find a way to get it done with what you have. There are still alot of old mechanics that use only crescents, vise grips, and a pair of Cee-Tee pilers.