Option #1: Scribble!!
Option #2: Background Options
Focus on Words or Text: Fill the background will various colors of text from magazines. Be sure to unify it by the way you lay them out or the size of the words/letters you use.
Use the magazines to create some sort of pattern. This can add movement to your background. (Try to think of similar colors within your pattern to unify the piece.
Monochromatic or Ombre magazine background:
Use the same color throughout your background OR go from one value of that color to another.
Take your magazine collage a step further by putting a layer of Gesso overtop. This will be translucent and have enough of a tooth that you can do a watercolor wash overtop to unify your color scheme.
Create a 3D textured effect with design or pattern using colored paper strips and paper quilling techniques in the background.
Coffee Filter + Magic Marker + water= Bleeding magic!!
This is awesome, you should totally try it!
Tissue Paper Patterns:
Cut tissue paper into similar shapes to create background patterns. This works best if you stick with a similar color scheme throughout.
Recycled Plastic Bags:
Scrounge up some plastic bags in various colors for a beautiful textured background.
Use maps to create a collage in the background.