I use my scraps. Actually find myself purposely buying more fabric than needed for a project just so I'll have leftovers. Most of the quilts I've made for Project Linus are scrap quilts. Strips mainly but many of the backs are bits and pieces sewn together into sort of a primitive crazy patch until large enough to cut into an 8 inch or so block. When I have enough blocks, I sew them together for the backing.
Anything goes. Solids, prints, plaids, stripes, holiday fabrics. I don't bother matching colors, just sew whatever fits the previous piece along one side.
The backs may not be beautiful but they are colorful and kids love to see what they can find. My own grandchildren have spent hours looking to see what familar item they can find.
Have also used them as scrappy sashing to go around machine embroidered red work kids designs to great effect.
Ellie