Old book pages are a great canvas to work with. We love adding botanical illustrations to worn-out book pages that have turned yellow over time.
Applying water color to old book pages in easy thing. The paper wrinkles when water touches the surface. But I learned a technique that painters use; stretch the paper after you run a water-soaked sponge over it.
We have a special Pinterest board for recycling paper, and it has a lot of artistic ideas. You can check it out here.