All these worlds are rife with resources waiting to be utilized. Europa, Ganymede, and Callisto are all cloaked in water ice, and Titan's nitrogen atmosphere is laced with methane. The Jovian Planets themselves are rich sources of hydrogen, oxygen, nitrogen, methane, ammonia, and other useful stuff. And let's not forget the sun itself as it contains 99.8% of the solar system mass, and ought to be good for something. Comets are good fun if you can catch them. Of course, with relative velocities in the hundreds of kilometers/second that's not completely easy, but hey, remember the applied determination.