My girl just loves space and planets and especially constellations.
Glow in the Dark Sculpey clay
A Baking Sheet
This one really couldn't be simple but takes a little time to prepare. Simply have you or your child roll out the clay and press it into a flat disk. Bake at 275 for approximately 15 minutes. I used my toaster over so I wouldn't have to heat my house up too much. Press the cooled disks onto the magnets (I purchased self adhesive magnets, but think I'll need to add some hot glue over time).
Then let the fun begin! You can print out cards like Laura from Lalymomr did, or you can just let them use their imagination. Since my daughter is quite young I opted for the second option. We talked about how constellation are like pictures in the sky. She told me hers was a mermaid.