Hi Lisa,
Usually just the experience of driving around dictates what you end up carrying....I once took an extra water pump, belts and P/S hoses on a trip from Vegas to Sacramento--on the back road--US 395, when you see a car about once an hour on the road (and I didn't have to use them)
I used to carry all my tools in an old, surplus ammo box, with the clamp-type top. You can get these in various sizes. If you got one, you could paint it yellow (or chrome it), and put a black scoop type GT/CS stripe on the side.
That would look kinda trick when you have to get something out, to fix something (a page from the street rodder way of doing neat things to cars).
These boxes lay on their sides, on top of the gas tank, or if it's a small one, you can wedge it under the spare tire.
But, then again some folks' only "tool" is a cell phone and AAA's number.
Paul N.