There are two main categories of squash - summer squash, and winter (or hard) squash. Almost any of the winter squashes will make good soup. So if you find acorn squash, red kuri squash, or pumpkin, give those a try. However, pumpkins come in two varieties also - those grown for size, and those grown for eating. The small pumpkins taste best. As far as substitutes - I think coffeesp00ns suggestion of carrots would be great, they will go well with the ginger in the recipe. I haven't made soup with sweet potatoes though, so I can't imagine what it would be like, but it's certainly worth trying. Call it a dessert soup. Sounds tasty!I had attempted to make apple-cranberry pie. ... I made too moist filling it collapsed into itself like a black hole