As others have said, use a backing fabric that has a pattern which won't show the seams too much and just sew it together. Another really simple way to spice up the backing is adding a simple strip of some other fabric or fabrics down the middle.

Here is the backing of a quilt my wife and I recently did. (actually we did two of them just like this). That strip is just simple blocks of fabrics from the front. Super easy to do.
Some people also use wide fabric from the fabric store, or sheets in a matching color although use of the latter is somewhat controversial within the quilting community.