Date, start time, mileage, ride time, route, max speed, average speed, weather, which bike, notes

Then I also have YTD miles and time, monthly miles and time, average speed for the month, average ride time for the month and other quirky things like average miles per week, graphs of weekely comparisons of mileage and time, etc. I also track how I'm doing per month against my year's goals.

I keep track of both mileage and time: mileage for the road bike and time for the MTB.