Keep a journal of your ride.
If you are not too concerned about weight bring along a small camera. You never know when a photo opportunity might arise.
I started a bike journal about a year ago. At first I only recorded distance, averages, and a general description of the route. After a while I expanded to general observations on my day. Finally, I started adding photos to the mix. At the end of the year I had a fairly detailed record of my experiences. I printed it out and plan to do an edition each year.
As you get older and start to forget everything, you can always refer back to the journal for clarification.