Anything you need tools to install, you can't use "in the field".*
First, install a Stowaway II grip. Everything you need can be put in a small ziplock bag and kept in the grip. I have done this on all my AR's.
I suggest:
1 Firing pin
1 Firing pin retainer (cotter pin)
1 Extractor
1 Extractor spring (with buffer)
1 Extractor pin (or two)
1 Trigger Spring
1 Hammer Spring
1 Disconnector spring
1 Trigger/Hammer pin
6 or more gas rings
-OR-
1 "McFarland" type ring (plus one installed on the bolt, in which case you probably don't need a spare)
1 Cam pin
The ejector and its spring and pin are included in some kits, but are not really "field replaceable" as you need a vice, punch and hammer to change the ejector. (Yeah, I carry those in my backpack... LOL) Not bad to have them, but you can't change them in the field.
Nothing else is likely to break or be replaceable "in the field". Now, at home, that is different.
* I am defining "in the field" as 3 days walk out in the woods or mountains, sitting by the campfire, or hunkered down somewhere, and working on your rifle on your lap with no tools other than a bullet tip.