I carry spare part to go to the grocery store. No not really, but everytime my wife and I go on a trip she says I take more time to pack things for the car than I do for myself. Here is a list of most of the stuff I take along and / or do before I leave.

check pressure in spare tire.
look at air filter.

stuff I carry
fuel filters just incase of dirty diesel
oil filter & enough oil to do a complete change
gallon of water
qaurt of trans fluid
qaurt of steering fluid
fuel additive
leather gloves
something to lay on OR a jump suit
a few rags

That's all I can think of off the top o'my head. Half this stuff stays in the car most of the time. It may seam a little much, but I've never been left sitting on the side of the road.