I use microsoft Word. The first part is quantitative like start time, total time, ride time, average speed, total mileage. But the most interesting thing is the commentary on the ride itself. This is where I describe my ride experience. You most likely have heard "listen to your body". Well this is where you reflect on the ride and express it. Later on, you can go back on this and figure out how and why you might have bonked, etc. I also discuss my new purchases, flat tires, helping other riders with flats, etc. during a ride, close calls with death on the road, etc.
I separate the bike log into calendar years. The important thing on the commentary is to do it right after the ride, say after showering when everything is still fresh in your mind.