Add chopped mushrooms, onions, bacon, white cheeses, cajun or red pepper spices to patties before cooking. Top with avocado, spinach, slaw, or fried onions for texture.
I bake hamburgers in the oven with sliced potatoes and onions thinly sliced. Layer onions and potatoes on top of meat on a piece of aluminum foil, place a lump of real butter on top, fold the foil over the stack, and bake at 360 degrees for 45 minutes. Tasty!