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.