I carry a large assortment of nuts and bolts. From 5/16's to 1/2". As stated usually longer ones rather than shorter ones. I also carry a couple of sets of drive shaft ubolts/straps&bolts as well as a set of leaf spring center pins.
Most things I have in a small parts box in the rear bed come from breaking (or someone breaking) something and my adding that combo prior to the next run.
I have broken drive shaft straps&Ubolts, spring center pins, used the spare bolts to reattach a TJ axle when the guy broke the upper and lower control arm mounts, steering boxes, spring shackle bolts, shock bolts, then list goes on.
A few things besides bolts that will serve you well, a few hose clamps, especially small fuel line sizes, axle ujoint half moon clips, some RTV, a length of twine, some black tape and of course the ever present duct tape.
Spring for a small 12"x12" compartmentalized storage box and go wheel. Then toss the few things in that seem to be needed when you are out. Soon you will carry around 20 lbs of small parts but will be Johnny on the Spot when "That Guy" didn't do any Preventative Maintenance or you actually need a leaf spring centering pin. (which has happened to me twice).