FK wrote:Why those three moons DiGs?
Titan: Titan is one of the moons of Saturn, and I'm pretty sure it's the only moon to which we've sent some kind of craft capable of taking surface pictures.
Here is a picture of the surface of Titan:

The reason I've always been interested in Titan is that it is the only other place in this solar system confirmed to have a liquid on its surface. The problem? The liquid isn't water, it's methane. But still, lakes, rivers and possible seas of methane and methane rain. . . . It's just a very interesting idea. Here's an artists conception that is just stunning.
EnceladusEnceladus is another of Saturn's moons. Here is a picture of Enceladus:

Notice how smooth the surface is? Considering other moons (even our own) it's incredibly smooth. It's surface is covered with water ice, and the fact that whatever craters exist are not visible suggests that there is some sort of resurfacing.

These plumes have been shooting out of Enceladus as long as we've been observing it. One of our spacecraft (Casini I think) flew through one of these, and was able to do enough analysis to show that it was mostly water.
So what happens? A lot of people think that just beneath the frozen surface of Enceladus lies a moon-wide liquid ocean of liquid water. Beneath the crust of the moon (aka the ocean floor) would be a molten core similar to ours, the circulation of which causes the oceans to move, giving the surface a geyser effect. Any craters get filled in with falling snow/water, thus allowing the planet to remain smooth.

Nobody knows what this ocean might look like. . . . . but might something like this be right underneath the frozen surface?

Artists conception:
Callisto Oops. . . I misspoke. Right planet, wrong moon. I meant
Europa!The reasons I would greatly enjoy a journey to Europa would be the same as I would enjoy a visit to Enceladus. This planet too is very smooth, shoots plumes into the air, and therefore is also likely ice covered.

The lines across the moon are vast chasms, likely miles deep. Miles of ice. . . what a gorgeous sight. . . It would likely look a good deal like Enceladus, but here is another interpretation:

Good?