A spare tube, tube repair patches, a chain break / repair tool with a master link, small compressed air canister (manually pumping a flat tire using the small pumps that strap to the frame will take a while, and fatiguing). A pair of clear safety glasses is a good idea as well. A rear cargo rack with a small cargo net will hold a bird for the trip back out to the gate. Everything you need to know about fixing a bike in the field can be found on youtube, tons of do it yourself tutorials to fix a chain break, repair a flat, etc.
Disc brakes are 100% worth the money if you are running rim brakes now, or considering buying a bike. Rim brakes tend not to work when wet, and going downhill on a skidder trail, in the dark, with no braking ability is a risk I'm not taking. Disc brakes are dependable regardless of weather.
Flat pedals with pins are also worth the upgrade - wide for boots, and the pins give you positive contact with the pedal.