My college apartment (will be moving back in a few weeks) has some pretty bare walls. While looking for things to spice it up, I came across this shop on etsy: WilsonGraphics
Check out the amazing beetle and butterfly decals he has that you can stick right onto your walls! These are some of my favorites:
They're generously sized (6" to 8"), very reasonably priced and will assuredly freak out any squeamish guests.