Don Elliott said:
Whatever spare parts you take with you will never be needed. That's the 1st Law of Murphy....
I'll second that!
I think I've told this story before: three weeks after buying my current Herald 1200 sedan, I took it on a 2,550-mile trip from NY to Minnesota and back. I had a respectably full tool box and some ignition and other small spares. I also had two minor problems:
1. A tire valve went bad during a lunch stop. Luckily, there was a tire shop about 50 ft. from the fast food joint where I'd stopped, and they replaced the valve.
2. My voltage regulator started sticking intermittently in Indiana, so I stopped at a Walmart and bought an extra battery, which I ended up not needing the rest of the trip!
If you know your car is in good condition, and you can trust it around town, there's really no reason why you shouldn't trust it on a trip. Sure, there are always unforeseen circumstances, but that's just as true with a new or nearly new car as it is with one 40-50 years old...like the air conditioner compressor that seized solid without warning on my '95 Mercury. Even my mechanic was briefly fooled into thinking that it was a bad starter...and then a seized engine...until he cut off the serpentine drive belt and found that the engine turned fine and it was only that compressor "locking" up the engine!
Basic hand tools, maybe a quart of oil...and your cell phone and road service card: that's really all you need. (And your gas mileage will be better with all the dead weight back home where it belongs!)