For replacement gear to use in the field, I've got spare boots, a spare sleeping pad, spare headlamp, extra consumables (gallon of water, couple dehydrated meals, spare IFAK with medications like antihistamines and painkillers, spare batteries, toilet paper), and an extra pair of sunglasses and ear pro.
For gear I only use at the car, I've got a spare sleeping bag (a 5lb flannel and canvas monstrosity that's much nicer to sleep in than the mummy bag I use in the field; I figure if I'm overnighting in my car I might as well enjoy it), a larger cookset, Work Sharp sharpener, folding saw, a more substantial FAK, and a couple Gatorades.
I keep everything I need for more serious weapon maintenance in the car too. If I'm using a rifle, that'll be a small driver with all the necessary bits including a torque bit, enough spare ammo to sight in with, a spare stick-on target, small bottle of Remoil, and a small tube of blue Loctite. If I'm using a bow, it's the same driver with different bits, a small bow press, spare string and peep, string wax, spare release, 3 spare arrows and 1 spare judo point for the incidental grouse, and a stick of hot melt glue.