Here are some of the "scraps" I put in stocks:
bones, skin, and gristle from leftover meats or chicken
chicken giblets
fish bones
onion, scallion, and leek tops
onion skins (will give your stock a very dark color, so be forewarned)
carrot tops
celery tops
pea pods
tomato skins
corn cobs
potato peels
egg shells
the ends of carrots, zucchinis, or any other vegetable where sometimes you'll cut off the tip before slicing
carrots, zucchinis, or celery that have gotten a little unappealingly limp but are otherwise still edible.
I keep a freezer bag in my kitchen freezer. Whenever I have scraps, I stick them in there, and then keep adding to the bag until I'm ready to make stock. Keep all the air pressed out of the bag.
I don't add leafy greens like spinach, or cruciferous veggies like broccoli, cauliflower, or kale, turnip greens, or collards, because I think that greens make a stock bitter.