I have three different spares carriers, depending on what's in them:
1. Tool bag #1: 12mm, 13mm, 14mm, 15mm box/open end wrenches plus 10-12-14-15mm sockets, Multi-tip magnetic screwdriver, compact spark plug socket and extension, 3/8" drive ratchet, pair of medium waterpump pliers (use as emergency crimp tool), several tie-wraps, spare spark plug (Gapped), distributor cap and rotor.
2. Goodie Bag #2: "Snail" carb sync tool, pair of walkie talkies (for group rides) plus their charger, 2 micro-fiber towels, Stop Light Prism (wrapped up in one of the towels), several pairs of ear plugs, LED flashlight, small leatherman tool, wine bottle opener, tire pressure gauge, dipstick thermometer, manual for the ridiculously complicated radio.
3. Spares bin under hood: Set of points and condenser (and my car doesn't even use them!), valve cover gasket, sump gasket, spare relay, roll of mechanic's wire, roll of 16ga electrical wire, long and medium length plug wires, coil wire (can also use a plug wire), small bag of nuts/bolts/screws (metric), small bag of crimp connectors, clutch cable, throttle cable, 1 each misc bulbs (except headlights). Car fuse spares are in my fuse/relay panel. Spare fuses (2 and unique) for the heater are in a labeled bag.
I also carry a small First Aid kit under the hood. The fire extinguisher is in the driver's door right behind my left elbow (heater and engine both have Blaze Cut suppressors).
I carry no spare tire (no room) so I have a 12 volt compressor and a tire plug kit.
I used to carry a windshield de-fogger (small sponge wrapped in chamois leather) but haven't needed that since I installed the heater.
I have used this stuff a lot over the years, usually to fix someone else's car (like at Carlisle), but at least I have the stuff when they need it.
The only thing I add for long-ish trips is a box with a complete distributor with all plug wires attached, already timed to just drop in and go.