I have attempted to keep such a notebook, though not necessarily as a diary. I recorded mostly comments or notes to myself about equipment experiments, gear lists or packing lists, and so on. Later I usually made a MS Word narrative saved onto a diskette. Those have been more useful.
I tried to note how far I went, both in getting to the trailhead and actual mileage on the trail, notes of the weather, brief comments about gear, if anything ususual was used, people I met on the trail and so on. The notes about the weather are more to put me in the place again rather for any scientific purposes.
Overall, the purposes of the notes are to help me recall the trip itself as an event, plus the details of the trip to help me with other outings. I try to have something experimental in nature to try out but I often go to the same places. So far I haven't been going often enough to get tired of them, though some places are much nicer than others and some are less visited than others.
I sometimes make an attempt to make a formal plan of an outing, which is only of limited value. I miss being about to go when I have the urge, rather than having to make plans weeks ahead of time. That's what comes of having other people in your life, I guess. The funny thing is, the harder and firmer I make plans for an outing, the more likely it will rain. I also doesn't help to live further away from good places to go.
One good thing about keeping notes electronically is the ability to be neat and to revise the notes as I have more time.
I never have done any artwork of any sort nor maps, though I often share comments about my outings (on other forums, however).
Another thing that is interesting is how different an outing is in person, if you follow me, than it seems like it will be as you sit in your room and think about it. It is either colder, wetter, nicer, or just positively wonderful when everything combines to give you a perfectly still and sunny day in the spring with absolutely no one else around.
