I track weight and mileage in an Excel spreadsheet. In addition to some of the stuff mentioned above, I calculate several averages (for example, since weight fluctuates daily, I calculate my 7-day average and 30-day average weight). Same with mileage - I watch 30-day trailing mileage.
I also track number of days I commute to work.
I don't track speed at all....my riding is so varied (flats. vs. hills; recreational vs. commuting) that I don't find avg speed very useful at all.