Stingy pork, shredded lettuce, sour cream, scrambled egg...all or some!
I've been eating them since I was a kid cos of my background Red kidney beans cooked as refried beans are the way to go - you'd find a recipe online for that - heaps of energy in refried beans and you can dispense with the meat if you use the beans.
You could fry up Haloumi cheese in thin slices and put that in, too.