When it comes to spares I've found that you'll need what you didn't bring and won't need what you did bring - . So I bring only the spares/parts that I know I can change between sessions or at worst overnight. If I'm racing at say Mid-O which is only a stones throw from my house I bring somewhat less.

I race a Ford so that generally necessitates more spares than say, a Honda , but here's what I generally bring:

1 set of wets
1-2 sets of dries (in case of flat-spot, puncture, etc)
Front knuckles with new bearings/hubs/studs - complete, ready to install
Front rotors
Front pads
Front Struts/springs/sways
Front calipers (loaded)
Pair of CV axles
Rear hubs/bearings - complete, ready to install
Rear spindles
Rear drums
Rear shoes
Rear shocks/springs/sways
Tie rods
Spare serpentine belt/timing belt
Spare belt tensioners
Misc hoses/clamps/tubing/tie wraps/duct tape/JB weld/bolts/nuts/screws
Necessary hand tools/drill/bits
Necessary wrenches/ratchets/torque wrenches/sockets
Necessary jack/jackstands
Necessary fluids/sealants/fuel
Small generator/drop-light/extension cords
Battery charger

Probably more that I can't remember........

Dan