I toast the bread first. Either cheddar or Swiss. Three or four slices, depending on thickness. I've occasionally used fresh mozzarella when I'm building one of my fancy ones.
My personal thing is to slice up and brown a couple of hot dogs and brown some chopped onions, then put a layer of those (a dog sliced into 16 pieces fits perfectly on standard bread in four nested rows of four) in the middle of the cheese. Ketchup on the side to dip it in as I eat.
Other times I use whatever meat happens to be handy - some chicken, a slice of ham, can of tuna, etc.