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.
Just remember, you don't have to finish your coloring page in one sitting. If you are looking to produce the best finish possible to your coloring page then don't rush it! Spend multiple sittings, or carve out a couple of hours, but just breathe and take your coloring slow. You aren't working to a deadline and there is no boss to please (other than you!). The only rule you have is to relax, lose yourself in the page and lay down some color.