A popular theme for notebook design/covers are famous quotes. Famous and influential people are often quoted on the front of a notebook – something witty makes a surprisingly good (and easy) notebook.
Other notebooks, put quotes throughout the notebook, perhaps a quote on every page or every few pages. One of my favourite notebooks from my childhood was a “little women” notebook which had quotes from the book throughout.
Quotes can also be used to fill notebooks. Some people have a “quote notebook” which they write down any inspirational quote they have heard and want to remember (there is even a iphone app for this). I do not personally do this. Sometimes I’ll write something in an existing notebook, but I do not dedicate a notebook for this.