http://www.annies-eats.com/2009/02/27/sausage-and-spinach-stuffed-shells/
Best recipe for stuffed shells I've found yet. We love them, and it makes a TON.
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 ricotta, mozzarella and Parmesan inside. I then put marinara on the bottom of the pan and on top, and mid-way through I put more mozzarella on top so it melts.
ETA: this sounds like it would be too cheesy, but I only put a little bit of the mixture inside, and it comes out perfect every time! It is really good.
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.