You have a fairly simple shaker cabinet, the countertops are a smaller more consistent aggregate and the floor is neutral. Based on all that I would look for a mosaic backsplash that picked up on the colors in the cabinets and countertops. This would add a "POP" to the kitchen. The one we used (American Olean 12-in x 12-in Mosaic Chateau Emperador Ceramic Mosaic Wall Tile, shown below) might work really well. We got it at Lowe's. It looks really good and best of all, it's only $4.98 a square foot.