I usually keep a log for all my outdoor activities. The main ones I write down are backpacking trips, boat trips, climbing trips, and kayaking runs. I haven't been that great about keeping up on the water related stuff, usually just writing down big trips, or the first time I ran a run. Usually I write down who was there, mileage in, pitches climbed, miles floated, gear I used, gear I didn't use, and complications. It is a good way for me to go back through and say to myself, ok, I used this, on this trip, or I wish I had this, or I have never used this, but I still pack it with me every time I go, do I really need this? It basically turns into an account of the trip, after I get some pertinent information/facts down, then I just start writing what I felt during the trip and what the experience was like.
Edit to add: I also am a big fan of writing in guidebooks. Especially guidebooks for climbing, etc. I will write down who I did it with, when, and just a line about what it was like. In the river guidebooks I write down campsites, things I notice on the way down, weird new sandbars, etc.
"What once was a river. Now a ghost." Edward Abbey