As an example and an inspirational starting point, I like to refer to the "Grail Diary" that Sean Connery's character Dr. Henry Jones Sr. kept in Indian Jones and The Last Crusade. You can find out more about this wonderful object at the wiki here. And here are a few pages from the diary:
Another inspirational example of a cross-curricular product- Leonardo da Vinci's sketchbooks:
I saw some other great examples at the Aga Khan Museum this summer:
Class Project Idea: For a summative project, students create a object in the spirit of the "Grail Diary" or a "Da Vinci Sketchbook" - that spans 3 or more subjects. The students can research their chosen subject through the lens of each discipline, and then compile the information into a comprehensive mini-book that demonstrates their cross-curricular learning. The book can be marked by each of the three teachers based on subject-specific criteria for their class, while appreciating the diversity of products and the depth of knowledge that results through this kind of learning and where the whole is greater than the sum of its parts.