Hi Amanda! A solar system theme will be fun!
The Space Capsule Craft on this site would be fun to do:
You could hang the capsules from the ceiling with the kids pictures above their desks/tables so they would always know where to sit.
This site has fun crafts, activities, dramatic play ideas, snacks, etc. There were cute ideas for making space helmets, space suits, rocket ships, etc. Many of the ideas you could use throughout the year.
I think it would be really neat to have a pointer with a star on it. If you use a glow in the dark star, you could turn the lights on during overhead time (if you use yours) and it would be neat for the kids to see.
ebay: Package of 700 Solar System Stickers for $1.25 plus .60 cents shipping.
There is also a solar system shower curtain (cute backdrop for reading corner/center), solar system floor puzzle, Blue's Clues solar system game (super cute), miniature NASA astronaut, etc. I'm a cheapskate, so I LOVE ebay!
Foam rubber pieces to walk on. Feels like you are walking on the moon. (At least my brothers and I use to think so!) They have lots of foam rubber at Surplus City, if you know where that is. It might be fun to have in the reading corner to sit on. Nice and comfy!
You might look for an astronaut costume for your dramatic play center.
I'll keep thinking about this. You already have a lot of great ideas and others have given some great suggestions.