When I was a kid, I had those cheesy glow-in-the-dark constellation stickers in my room. They looked so cool when the lights were off, but the stickers were all over the furniture and walls, and looked silly during the day.
I saw this Star Wars Death Star Planetarium in an email from Chapters/Indigo, and I thought it was so cool. During the day, it looks like a replica of the Death Star (not as scary as it sounds – basically a grey globe), and at night, you can take the lid off and it will project stars on your walls and ceiling. You get the “sleeping under the stars” effect, without doing any permanent damage to your walls or furniture (seriously, those glow-in-the-dark stickers were a pain to remove).
Want something a little more cute and cuddly? There’s a stuffed turtle that does the same thing.