I want to see Pluto and its moon Charon. Can you ever see them?

No. They are visible only in the largest telescopes and then only as dim dots. When the Pluto/Charon system passes an easily located object, you might be able to detect it, but it would not look good. Pluto was discovered photographically using a specially designed wide-field camera.

Why does the gravity on planets vary? I heard that if I were hovering in the cloud-tops of Jupiter, I would weigh over twice what I weigh now.

The local strength of gravity depend on two things: how dense a planet is and how massive it is. Jupiter really doesn't have a lot of gravity at its surface for something over 300 times as massive as the Earth, but that is because its not nearly as dense. The Moon, on the other hand, is almost as dense as the Earth, but not as massive, so it has a lower gravitational force on objects. This is the reason the astronauts hopped around like deranged bunnies. They weren't accustomed to such weak forces.

Are there any planets with atmospheres that we can breathe, other than the Earth?

Not in our solar system.

I remember in the 1950s, there were all these science fiction novels that had Venus as a rainy, jungle-like, planet. Now I hear that Venus is hot and terrible. Didn't people know back in the 50s?

The temperature of the dark side of the planet was measured in the mid 40's with a device attached to a telescope called a bolometer. Venus was found to be hot by this measurement, but that fact didn't achieve wide distribution. What the science fiction authors were doing was basically wishful thinking.

Why does Saturn have rings and the other planets don't?

Actually, other planets do have rings, but not clearly visible rings like Saturn. The reason for Saturn's rings is that the planet Saturn has a complicated moon system that acts to slow the natural dissipation that would occur if they weren't there. The outer moons are fittingly called shepherd moons.

Is there any life on Mars?

That is basically an open question. There were once flowing rivers on Mars, but whether they were unfrozen long enough for the establishment of life is debatable. Suffice it to say that there are places on Mars where some microbes could survive, but it would be a very hardscrabble existence.

Is Mars the best planet to look at? I remember reading that it was the closest.

Mars is too far away much of the time to present a good view. It is also very small. Every two years and a few months, it passes reasonably close to the Earth. It then gets a little bigger than the disk of Saturn. Most of the time, Jupiter and Saturn are the two best planetary views. Venus would be good if it didn't look like a cue ball.

I saw back a long time ago where a comet hit Jupiter. Could that happen here?

Yes, but because of Jupiter, it's a lot rarer than it might be. Comet Shoemaker-Levy 9 entered the solar system and got attracted to the biggest planet around, namely Jupiter. Jupiter has probably taken a lot of the heat off Earth.

The two films Armageddon and Deep Impact, were about the impact of comets or asteroids. Would the best solution be to blow them up before they reach the Earth?

The best solution is to discover the interloper as soon as possible, preferably decades before impact, and to gently move it aside into a disposal orbit where it will interact with a second planet. Thus it is thrown out of an Earth intersecting orbit. (It can get you in the second pass as easy as the first!) The longer you wait, the harder it is to do anything about it. Nobody will blow it up in the last seconds. Last minute fixes like this would cause Earth to be slammed by a few big rocks instead of one huge one. It would make little difference.

I read a book that said the Earth was hollow and that there was a little sun inside the Earth. I heard that an arctic explorer discovered this fact. Is this possible?

No. Three reasons:

1) Our ability to locate earthquakes is calibrated for a solid earth, and the procedure works quite well.

2) The little sun would be in an unstable configuration; it would eventually move against the sides.

3) There is no known mechanism that would support the interior walls. The Earth at that size scale resembles a bunch of slush. It would just fall apart.

Well, I heard another hollow-earth theory that didn't have anything to do with that story. In the hollow-earth theory I heard, we are all really standing on the inside of a spherical universe, and the galaxies as you go toward the center all get smaller and smaller. Can this be true?

There is nothing wrong with that theory, as long as you are willing to live with the complicated equations. What this theory is based on is a point-to-point mapping from the outside world to an inverted sphere, kind of like saying the reflection in a Christmas-tree ornament is the true world and that the only reason that we think otherwise is because of our perception. There is no reason why this can't be true, since a point-to-point mapping can also be done to all the equations in the universe. However, those equations, which in the solid-earth world may be very simple, like "light moves in a line along a geodesic" become horribly complicated in the hollow-earth world. Science prefers to follow the principle of Occam's Razor, which says that in the case of two equations giving equally good results, the one that is assumed true (unless contrary information is uncovered) is the simplest.