Keeping a Vacation Log
I've kept track of where I spent the night, what I had for supper, then where I went for breakfast, maybe even what I ate. I remember one night back in 1984 during my solo drive to Alaska--my first night on the Alaska Hwy. in a campground in British Columbia. I was invited over to the campfire of several couples from the Midwest who were traveling together. They shared their coffee with me before I went off to my tent. I found up there that, unlike in the Lower 48, there are no fast route and scenic route between point A and point B, but just one route. So, the next morning I drove a few hours up the road to a hot springs, where I found them. The next day I was at another spot, and they came in just after me. The third day, I stopped at a pulloff area to make lunch, and they honked as they drove by again. If I'd not kept a log, I'd have probably forgotten many of the details of that experience by now.
Another reason for keeping a log is to record details that you won't necessarily key into your computer when you get home for printing off and putting into the finished product--the booklet-type log, possibly--but which could help you differentiate between the 3 waterfalls you saw in one day. That sort of thing can be recorded at the back of the notebook, well beyond the actual log portion.
As already alluded to above, people you accidentally meet along the way can quite often be a big part of the trip. Say you were at a campground, in the hotel restaurant, or at poolside on your cruise ship, and you meet Hiroshi and Suki Yakamoto from Japan, and start talking with them. You even ask the waitress to take a picture of all of you. But, you neglect to record their names and where they're from. Then you get home several weeks later and come across the picture, wrack your brain for their names, but finally have to settle for merely "a Japanese couple we met" because the details weren't recorded right then and there.
Also, as opposed to keeping little trinkets like attraction receipts, youth hostel passes, fancy restaurant napkins, etc. in your photo album, they can be kept in your log book. I can't stress enough how much this can mean to you years later. An alternative to keeping them loose would be to get them laminated, in case they're the type of thing that could be easily damaged over the years.
In conclusion, you'll never kick yourself for writing too much down, but you certainly can do so for not recording enough. Don't let it happen to you.