My little guy just learned the names of the planets in the solar system...I never took it farther than that, but mostly because he didn't care to learn more than that... I found some song at one point that went through all the planets. It even went over that Pluto is no longer a "planet".
We made solar system cookies (sugar cookies) to scale. Earth, Mars, etc are quite small - about 1" - and cook really fast! The gas giants are considerably larger. I got the idea from a homeschooling magazine at the library. You can't make a sun but you can talk about how big it'd have to be (ours was as big as our dining room). We also frosted/decorated them with candies and laid them all out on black plastic. We made an asteroid belt from Nerds candies.
We also sing the Blue's Clues song - probably on Youtube ("Oh the sun's a hot star/Mercury's hot too/Venus is the brightest planet/And earth's home to me and you...")