My professor gave us some insight with notebook recording as well, and it's definitely given me a new perspective of the use of notebooks. She's been going over phenomena when students need to record down their observations, insight, prior knowledge, generally anything that can be accounted for to demonstrate the engagement within your students. So activities that involve observational skills and the use of their five senses would be a great start to creating their notebooks. For example, an activity where the students partner up and need to use the five senses to describe to their partner what fruit it is without actually saying the name of the fruit.
If I were to organize the enteries, I would base it on the standards you wish to touch on as the school year goes on. For example, divide the notebook pages based on the NGSS topics, which is life science, earth and space science, and physical science, then fill each section in that relates to that topic. So, if you have 100 pages in your notebook, approximately 33 pages will be sectioned off for each topic and the students can write in their enteries with the designated section.
Hopefully that makes sense, but this is something I would like to try when I have a classroom of my own!