Last time I made them I mixed the hamburger (seasoned with garlic) with ricotta a fresh parmesan and motzeralla and stuffed that in the shells. Then I topped with sauce and more parmesan and motzerella!
i put a mixture of ricotta, garlic, salt, pepper, and a little shredded mozzarella inside. sometimes i add spinach or mushrooms if i want to. then i top the shells with marinara and more shredded cheese. you really can put whatever you want in them. maybe broccoli or meat too.