Here's a fun way to add a collage_like feel to your coloring book page: try make a rubbing! All you have to do is put some sort of textured object under the coloring book page (above, it's a series of coins). Then gently shade the space with the side of the pencil or colored pencil point to fill in an area. It can give interesting bits of texture to an area in your coloring book page, and when used on an entire page, it can create a cool effect.
Make it a zentangle. By using ink, you can "color" in the page using different pen strokes like hatching and cross hatching and tiny patterns to create an intricate coloring wonderland. Before you know it, the page will completely change character, becoming almost like a collaborative work between you and the coloring book page illustrator.