In addition to the cheese, crisp bacon slices and/or 1/2 cup of well browned chopped onion. They're delicious without either, though. Try them plain first.
If you're in a hurry, you don't even have to flour and fry them; just put in the cheese and return them to a pan with a bit of oil in it, and cook them till you know the cheese is melting. That's good, too. The coated ones are simply better. If you wanted to get more elaborate you could dip the peppers in beaten egg and then the flour, for a puffier, more substantial crust. Not necessary, though.
Queso asadero is a type of Mexican cheese that melts well. You can substitute any mild, melty cheese. You can substitute Monterey Jack cheese.