I use an Excell spreadsheet with days in columns with a weekly summary column after every seven daily columns. The rows are:

Day
Date
Daily mileage quota (according to a ten-week century training program I use to prepare for a century, and for maintainence)
Excuse (really )
Distance
Time
Average speed
Average Cadence; these last four items from the computer
Difference (Quota-Distance)
Route
Arrival Time (mostly for commute to work)
Comments

The weekly columns are total mileage for the week. I then graph my actual weekly mileage in comparison to the weekly quota (as side-by-side vertical bar graphs), and I graph my daily mileage similarly with a secondary line graph of my daily average speed. OK, I admit I am a wonk, but it's very motivating to see the display and gratifying to enter the data.

I used to also record some things like resting HR, temperature, wind speed and direction (at the start of the ride), mileage at which I spontaneously feel warmed-up, preceding sleep length; but these all fell by the wayside, and may be recorded as comments. For wind speed instead of mph, I use my own version of the Beaufort Wind scale depending on which way and how the flags are flying en route, 0 to 6(+) for tailwinds and 0 to 6(-) for headwinds. A flag waving almost to straight out is about a 3(+) or 3(-).