I think the bigger question is what WON'T I put in soup :) Seriously, just about anything is fair game! I use my crockpot alot for soups: Taco Soup, Minestrone, Chicken Tortilla Soup, Chicken Noodle Soup, Chicken Enchilada Soup, Chili (both white and red, both chicken and ground beef/turkey), Chicken or Turkey Wild Rice, Ham Bone and Cabbage. Cream based soups I do on the stove: Clam Chowder, Cream of Cauliflower/Ham (used to be potato until I figured out I could use mostly cauliflower instead and no one would notice), Cream of Mushroom, Seafood Chowder, Cream of Tomato and Basil with cheesy tortillini.
I like using Cannelloni beans, black beans, and hominy in soups. Spinach, kale, and leeks are great add ins. Any meat (except maybe kielbasa) I think I would use without reservation. The only veggie I think I haven't used in a soup is probably brussel sprouts. When I make a soup, I always poke around the fridge and see what I have laying around to throw in - if in doubt, I always saute it first (I think onions, green peppers, celery, and other veggies taste better if I fry them up first - just a personal preference). One thing I wish I had constant access to is parmesan rind. I would use it in most all of my soups if I had easy access to it! I throw it in along with the rest of the ingredients in the crock pot and pull out and throw before serving (like you do a bay leaf).
Spinach is a personal favorite! I bet you would love a good Minestrone :)