"What, if anything, do you use to document when and how you use your tractor? Something that wold include the Date, Hours Start, Hours Stop, Task, etc. "
I keep a small logbook on the work bench right by the tractor. I log date, hours and comments (i.e. Grease job, oil change).
PC is OK, as long as you actually remember to log your work/maintenance after doing it. Although I love Excel, I think it's overkill for the task.