Wall art -
Who needs wallpaper when you have a sharpie? As seen on Design*Sponge, you can create your own wall texture with just a flick of the wrist, or rather several flicks! It would work great with a stencil, since you won't have to worry about it leaking and ruining the pattern!
Shirt options -
Since sharpies don't wash out - they are great for making shirt designs - you can create your own drawing on a plain shirt, add a little something (such as an environmental slogan) to an already designed shirt, or try this modern twist of TieDye.