For some reason I really like a stripe fabric for backs. I would look for a stripe that had some of the colors from the front.
as another idea, I would make some additional blocks like you have on the front and run a row of them either horizontally or vertically on the back and use a complementary color or stripe on either side of the row of blocks. When I piece a back like this, I set the row of blocks off-center in a 1/3 and 2/3 ratio.