I bring a box of washers, nuts, bolts, and various other little stuff (fasteners, e-clips, springs, kingpins, etc). Sometimes it seems I’m the only one out there with a decent bolt collection.
Also, all the tools you’ll need. Li-ion impact, and a portable toolbox from craftsmen are what I use. Also a box with all the parts cleaner, wd-40, oils, and lube you’ll need for the day. I also bring a spare axle and spare front end stuff (spindles, tierods, kingpins, steering column). Also spare chains (already broken to multiple lengths) and spare master links.
To me having all the tools and little odds and ends with the bolts and washers is probably the biggest thing.