- Animals (could be broken into 'safari animals,' 'ocean animals,' 'farm animals'...)
- Foods (might be an interesting way to get them to eat more veggies!)
- Places (going to the beach? why not snap pictures of what to pack or what you'll see?)
- Seasons (what you need and what you can do during the Winter, for example)
- People (live far from family? why not show pictures of family members with their names so the kiddos are already comfortable and familiar when you visit?)
The obvious cost here is in the production of the book, but it doesn't have to be that expensive! Websites like http://www.snapfish.com/ offer flipbooks (from $4.99) to hardback books (from $11.99), but if you have a color photo printer, print the pages yourself and take them to Kinkos to be bound into a spiral book with a plastic report cover and thick-paper backing. The benefit of having a company print the book for you is that you have many more options for layouts (with drag-and-drop simplicity), but you also have to factor in tax and shipping into the price.
Here are a few pictures of what your book could look like: