Create a design on a t-shirt using a bleach pen. The pen, usually used for taking stains out of white cotton clothing, can be used to deliberately take dyes out of colored cotton clothing. So grab a t-shirt, pick up a Clorox Bleach Pen for Whites (important, that "for whites" part!), and create a design in less than an hour.
Find directions here:
I found that putting a big piece of cardboard inside the shirt before using the bleach pen helped stabilize the shirt and didn't allow the bleach to seep through to the back of the shirt.