Bolts from competent manufacturers
Extractors
Extractor springs with inserts
Cam pin
Ejector spring/selector detent spring (same part, every standard AR-15 has 2 inside)
TDP Firing pin (checked for protrusion spec)
Firing Pin Retaining Pins
TDP gas rings, not imported trash
You can also just have spare BCGs that are east to plug-and-play.
Bolt disassembly tool or use the spent case method with a vice. Bolt disassembly tool is really small and light though.
Spare action/recoil springs of correct material and length per the TDP
Lower Parts Kits
Spare Fire Control Groups
Carrier Key and fasteners
Rockset
Red Loc-tite
Spare gas tubes and roll pins
Gas block small parts for your specific gas block, whether taper pins, set screws, pins, or fasteners
Spares for your specific optics and aiming devices, such as batteries, ring fasteners, caps
Tools to replace as much as you can from above.
If you’re in an extreme cold climate, consider having a spare pistol grip if you went with an after-market option. I broke a 1st Gen MIAD in Finland doing CQM drills, suppressed, in -30˚C, 3-day course shooting high volume. That grip just cracked in about every way it could. Real A2 grips never did that on me. Magpul changed the design to use a flat screw instead of the original conical screw, but the little tabs that hold the backstop and grip core in place completely fragmented, in addition to the grip itself splitting. I had a spare grip in my rifle case in the range building/barracks where we were staying, so I was able to replace it and continue training.
Build a compact kit in a Plano case you can have in one location that’s easy to grab and pack.