Here are some common things you might find in a diary:
Personal Thoughts and Feelings:
* Daily experiences, both big and small
* Reflections on events, relationships, and emotions
* Hopes, dreams, and aspirations
* Fears, anxieties, and worries
* Personal opinions and beliefs
Daily Activities and Plans:
* To-do lists
* Schedules and appointments
* Record of conversations
* Memories and anecdotes
Creative Writing:
* Poems, stories, and other creative work
* Sketches, drawings, and other artwork
Other Potential Entries:
* Recipes
* Travel plans
* Ideas and brainstorming
* Journaling prompts and exercises
Ultimately, what's in a diary is completely up to the individual who writes it. It's a private space for personal expression and reflection.