What would Jupiter look like from a Galilean moon? If I were on a Galilean moon of Jupiter, like Europa or Ganymede, what would the planet look like? (Let's say I'm on a space station that provides atmosphere to make it slightly less unrealistic.)

Would it look like our moon, only a bit bigger and colored like Jupiter, with the stripes and the "eye", or would the moons be close enough for Jupiter to look gigantic and fill a significant portion of the sky?

The two moons that are closest to each other are Io and Europa. They seem to be about 250,000 kilometers away from each other. Would it be possible to see one from the other with the naked eye if they were aligned? For example, if Europa were in a position in its axis where it is lit by the sun, could I see it, looking at it from Io? Or would I need a telescope? A 3000 km sphere from 250,000 km away does not seem possible. The other way around, of course, Io is a bit bigger.

But the part I'm the most interested in is: what would Jupiter look like from one of these moons.
You can simply startup Stellarium and have a look yourself. Choose any locations you are interested in. The Galilean satellites of Jupiter are inside the default list of locations.

The attached image shows Jupiter as viewed from Io at the given time, Europa is the bright object to the right; with -9 mag it is considerably brighter than Venus when viewed from Earth, but slightly less so than Earth's Moon as seen by us.

Interesting to note is also the huge extend of Jupiter in the field of view, if you choose the right hemisphere (Io is in a tidally-locked state). With roughly 15° you need about your entire hand at arm's length to block it while the sun is a lot smaller (~30x larger than Sun or Moon when viewed from Earth). Also the daily eclipses of the Sun by Jupiter might be noteworthy.