Concentrate on specificity and depth. If your writing has a focus it will be more interesting for you and your reader. The proposed structure of the Field Book will play an important role in helping you stay focused. A successful Field Book will require both organization and creativity. The above mentioned structural requirements do not preclude playfulness and creativity within the basic framework.
Writing is a process and has different stages.
Later, as you write your Field Book assignments, you may develop and expand these rough ideas. Brainstorming about details is also part of drafting. Both these techniques should precede your first draft. Don't look at sharing as an invasion of your privacy, think of it instead with expectancy about what you can teach and learn in the process. Reading your work aloud to others helps you internalize the idea of an experience.