I recommend that each time you do a formal nature study that you choose a topic to focus on. You should vary the topic throughout the year, but feel free to revisit some of the topics several times in a year. For example, your children should choose a tree to observe throughout the year. Your nature study for at least four times in the year would focus on observing the tree, once in each season.
You may want to take a look at the sample of our Journaling a Year in Nature book for an example of how you can change the focus of your nature study each time you go out.
In addition to your more focused nature study time, you should allow time for spontaneous nature study too. Sometimes unexpected opportunities to observe an aspect of nature are the most memorable learning experiences!