I also discovered the little hats that come with Mr. Potato heads fit my 4G iPods! I don't have a 5G, so YMMV. I've included complete instructions for the ghost outfit, plus a pic of my iPod as R2D2 after the jump. Feel free to share your own creations!
For the ghost costume, you're gonna need a marker, tissue (or other malleable white material, a Kleenex just happened to be handy-- I use it to wipe my Mighty Mouse's nose). I peeled the tissue ply so it was bigger, and covered the entire iPod. Then, I sort of pushed down a bit, to form the paper to the iPod. Making note of the screen area, I removed the tissue, drew the face, and re-applied. Voila! I don't have any handy, but it would seem a dry erase marker could be used to draw directly on the iPod. Given past experience with dry erase though, don't leave it on there too long! (And TUAW assumes no responsibility for your now-discolored iPod).
And here's the pic of our fun little droid iPod... Wouldn't an R2D2 iPod be great?