Painting tee shirts can be a great medium for something creative, but actually usable/wearable. Stencils are great for this, but sometimes it can be finicky to cut out the inside of a stencil using a Stanley knife.
Here is an easy tutorial on how to stencil by simply cutting around the outside, allowing you to avoid difficult cutting techniques. The paint will go around the stencil, leaving an unpainted relief as the main motif. I did this one as a gift for my friend who loves cycling.