I keep one too! Sometimes not as detailed as id like, but I always try to make some basic notes. I always note the date, location, and weather, along with what I see. I also try to note times in and out of the woods, sunrise or set. It's helped me remember what time I need to get upon the morning on more than one occasion! I've always liked to draw so I might make a sketch of anything I find interesting and then put my notes, observations, and thoughts in around it. I use a small moleskin book without lines. Not cheap but its sturdy and will last for a long time!