One of my absolute favorite teachers to follow and gather ideas from is Dr. Kristie Meeuwse, a kindergarten teacher and Apple Distinguished Educator. She has so many wonderful ideas for meaningfully using iPads in her classroom. Math journals in my kindergarten classroom were inspired by her, and her latest blog post talks about how she uses them in her classroom. It's definitely worth a read and has some great examples. She uses the app Book Creator ($4.99 but worth it) to make her journals.