The asteroids, especially mineralogically-rich asteroids
Let’s get one thing straight right away: depictions of the solar system’s main asteroid belt as an obstacle course for spaceships is bad science. On the scale of a football field, all the asteroids in the solar system would be a minuscule pinch of dust concentrated just a few yards from one of the end zones. You’d have trouble finding the asteroids, let alone crashing your ship into them.
But that doesn’t mean they’re not important, or a cool place to set a story. Mining the asteroids for important minerals and perhaps water might not remain fiction for too much longer, and there are loads of stories that treat this idea. One of my favorites (though perhaps not the most realistic) is R. A. Lafferty’s “Golden Trabant,” that tells of the discovery and subsequent loss of the legendary El Dorado of asteroids, an asteroid of solid gold. (Hey, it’s possible. Maybe.)