Backgrounds can just be a wash of color. Also to get a darker level of shading, you could "dust" the area with a softer lead or charcoal. So basically, you take a cotton ball and rub it on charcoal or you may need to shave some dust of the charcoal and then pick it up with the cotton ball. Then use the cotton ball to apply the charcoal to your drawing. Go back with an eraser to pull out highlights or areas you don't want to shade. I also use this method just to add "surface" quickly to a sketch.
Here's a quick sketch in which I used this method. I shaved off some color pastels and applied with cotton.
You could also cross hatch the area you want darker with a softer lead and use a burnishing tool (or tightly rolled toilet paper) to blend it in.
As for backgrounds you can always do a wash of color or abstract pattern like this.
*not my work*