Yep, anything good tasting!
Onions are very important in a soup, don't make one without first sauteeing a small chopped onion in olive oil. :)
Then you can have a traditional soup with lots and lots of thyme, bit of sage, garlic, and parsley.
Or you can have a more spicy soup, by using equal parts powdered ginger and cumin, and half part spicy paprika. Very yummy! Don't put garlic in this one, doesn't need it.
An Asian soup would be equal parts soy, rice wine vinegar, with 1/3 the amount of sugar. Add things like sesame oil, roasted sesame seeds, seaweed, and hoisin sauce.
Try the organic chicken or veggie broths that are now in cartons, they're quite good. :)