-Chains of pool noodles
-balloon punch balls, grouped in 3's to represent molecules.  (These work great because they are large and sturdy).
-spiral, plastic windchimes
-colored plastic slinkys
-colored wiffle balls

It may not sound like much but these items do look "science-y" and spacey!  And when combined add a lot of color and texture.

We also found spiral twirlies at Dollar General in the toy section for $1.  They came on an orange plastic stick and we simply popped them off the stick and push pinned them into the ceiling.  They are very colorful and coordinated well with the other items.