If you're having trouble coming up with ideas, do a written version of a free association exercise. Just start writing whatever pops into your head. Usually, before long you'll have a number of topics you could write about. To give your entries some length, make sure you give plenty of detail.

For example, if I were going to write a diary entry for today, I'd probably focus on the shopping trip my wife and I took. To give it proper perspective, I'd briefly discuss how we talked about it first--we didn't really feel like going and wanted to stay home and watch a movie instead. But we figured out that between work and other obligations we really only had a couple days left to finish our Christmas shopping. So we figured we'd better go. Then I'd narrate the experience. The lines, the frustration of not finding what we wanted, and the fatigue we felt after going to several stores and fighting the crowds. Finally, I'd finish by telling how much better we felt on the way home when we picked up a couple of pizzas--they smelled so good in the car on the way home!

See how you can take a simple subject and give it scope and background? I'd probably get about a page and a half out of that.

Have you read The Diary of Anne Frank?  Take a look on Google Books if not, and you will see the concept.  Often people use a diary as a "release" book in which they write their feelings and observations--a form of written introspection.

Take a walk and your thoughts will flow as you will automatically switch to your right brain. Perhaps, you may even wish to carry a small notebook with you and jot down ideas that emerge as you walk.  These can be about nature, your feelings, you continuation of a thought started earlier that day--whatever.

A diary is a wonderful way to vent feelings without hurting anyone, as well. In fact, psychologists suggest that people use diaries or letters to crumple and destroy if they are angry at someone.  Never sending the letter is the idea, but venting is the benefit.

First, you should understand what a diary is. It is generally defined as a daily record of events and experiences. Some diarists start each entry with the description of the day's weather. Korky on "Murphy Brown" always began her diary entries with the weather and the description of her outfit for the day so as not to repeat an outfit too soon .... Diarists make single page entries that list particular events of the day, either mundane or out of the ordinary, from doctor appointments to excursions to heartbreaks. For may of us, days go by rather uneventfully, so we think there is nothing to diarize, yet if we take a look around, even our dull days hold surprises (whether good or bad) that are worth being noted. Some examples are an algebra class substitute who did nothing but read a romance novel, which made you wonder about the "teacher" part of "substitute teacher"; a classmate was humiliated by some thoughtless and unkind text message that went the rounds of the school; your mom brought home your favorite ice cream as a surprise.

I would suggest choosing a topic where you can use a lot of description.  For example, it is holiday time for many.  I can clearly remember Christmases past with people who are no longer with us.  Choose something meaingful to you.