I'm sure no one needs a "recipe" for hamburgers, so this is more of a fun idea. We had some friends over for dinner and had a "make your own burger" party. We had a lot of different toppings to choose from:
cheese (two diff. kinds)
I do add worcestershire sauce in the beef before grilling, it adds a yummy flavor (my kids can't even tell it's there) and tenderizes the meat a little. I've heard soy sauce does the same thing.
We do the "make your own" dinner quite a lot at our house...pizzas (on Greek flat bread), burritos, tacos, etc. My kids love the idea and eat a lot better when they get to assemble it themselves.