There are all those uses listed in this thread, most of them very predictable: journal, sketchbook, memoir, planner…
But does someone have something really extravagant? (I don’t.)
Juan Kerr says:
How about turning a large plain Moleskine into a magazine? Pick a topic; films, music, art… whatever, and away you go. You’d be editor, writer, layout designer, publisher. You could snip small pictures from a regular magazine and paste them into your Moleskine, alongside your articles and features. There would be an awful lot of work involved, but imagine how amazing the finished Moleskine would look.
Photo: "skines" by SlipStreamJC