Here's an artist, Mama Jamie, who decorates all kinds of things including a guitar, a piano, and body parts (with henna). The guitar is interesting, but I like the piano better. I don't think you can do much damage to a piano by painting it.

I think if I wanted to decorate a guitar, I would want a Celtic knotwork design, or maybe Aztec, or maybe Northwest Indian. Might as well stick with a traditional motif. (Those images aren't guitars, they're just examples of traditional styles I happen to like.)