The green parts of leeks can be sauteed, and are very tasty. Be sure to rinse them well, to get rid of all the grit. Slice them thinly, and saute in oil or vegetable broth. They're very good with onions and garlic.
You can also toss them into the pot when you're making vegetable stock.
Remember that the white part and the green part have different textures and cooking times, so they probably shouldn't be mixed together to cook all at once.