My favorite kind of risotto besides a simple cheese risotto is mushroom risotto. I was craving duck too so I served the two together. For the mushroom stock you can use scraps or button mushrooms. Just combine equal parts (by volume) of mushrooms and water and add a bit of salt. Bring it to a boil and let it simmer until you get the flavor you want.
The general rice to liquid ratio for risotto is 1 to 3 but you can always keep extra mushroom stock and play around with it to get the consistency you want. I seared some mushrooms and folded them into the risotto (with grana padano cheese too)