One summer when I was ten or eleven-ish, I had the nicest babysitter. In addition to a daughter my age, she had a petting zoo of farm animals (I remember the ferret and the pot-bellied pig, in particular), an unlimited supply of Disney movies, and a trampoline. Also, as long as the messy parts were done outside, she was very encouraging of our art projects--I believe that was the summer I learned what a hot glue gun is.
One of our favorite art projects was a game called "T-shirt factory". A box of puff paints, as many of my Dad's white undershirts as he'd let me steal, and we were in business for a whole afternoon of t-shirt decorating fun. So when a co-worker gifted me a box of her son's outgrown clothes, I knew exactly what to do with them.
I decorated these with freezer paper stencils. While you could easily draw your own, I used some from the Lotta book. Although you can't see it in any of these pictures, it's very important that you put some sort of barrier (cardboard, a grocery sack) inside the t-shirt, so that the paint doesn't bleed through to the back.