Here are some possibilities and ideas to ponder:
Types of Diaries. While most diaries are about the day-to-day events of someone's private life, there are other kinds of diaries too, such as travel diaries.
A Story with Multiple Entries. The best kind of diary page uses multiple entries to help pace the story, providing a sense of drama and anticipation as one events after another takes place, with each event getting its own diary entry.
A Story with Multiple Diaries. You can also combine diaries of different people in a single page so that their different diary entries tell the same story from different points of view.
Animal Diaries. It's not only humans who keep diaries; you could imagine animals keep diaries too!
Finding the Diary. If you are thinking about diary style for a Storybook project, you might also ask this question: just who has possession of the diary so that we are able to read it? Is the diary something passed on after the writer has died? Was the diary stolen? Thinking about just where the diary (or diaries) come from can be a frametale that you tell in the Introduction.
Here are a few diary projects: