Make the base with at least 2 of these: leeks, onions, celery or fennel.
From there, add any green vegetables: spinach, kale, swiss chard, broccoli, green beans, diced zucchini or anything else in season. Frozen vegetables like peas work great too!
Just be sure to adjust the cooking times to match the vegetables you use. Add the veggies in phases, from longest cooking first to fastest last, to guarantee the ideal texture for each.
As the weather turns colder, potatoes, white beans or cooked pasta would make it for a heartier soup.