I usually carry:
1. spare distributer cap with wires
2. a generic radiatior hose
3. fan belts.
4. Because I've broken one before (and no body else I've ever met has), i also carry a spring center bolt and a couple of U-bolts.
5. Spare brake line
6. Spark plugs
7. fuel hose
8. Air filter
9. Fuel filter
10. Miscelaneous nuts, bolts and fasteners
11. Fuses and wiring stuff
12. The tools to install this stuff.
I know some people carry spare locking hubs as well, but I've never broken one before.
You can go crazy trying to carry everything. I usually save my parts from servicing, and any parts I upgrade, and keep one spare with me from what's left over. Also, anything I know has a tendency to break I'll carry a spare one.
Hope that helps
Erik