I picked up a new hobby! Freezer paper stenciling. All you need is an iron, some acrylic paint or fabric paint, some freezer paper, a knife, and an idea. There are dozens of great tutorials online.
I was surprised and delighted by the crisp lines I could achieve. However, I have learned that very thin parts of the stencil got muddled when I blotted on the paint too thickly, as you see with the orange text on the Faster Than Light onesie. Perhaps I can clear up the lettering with white paint and a thin brush.
Seriously though, if you ever want to decorate a t-shirt/bag/onesie/etc., try freezer paper stenciling!