Have your child design and create their own t-shirt.
This activity really works well with inexpensive white t-shirts you can buy pre-packaged. Just make sure they're 100% cotton.
If you use markers, be sure they're the permanent kind. If you choose those, the child will draw directly on the shirt.
Note: Be sure you place a piece of cardboard or some plastic directly inside the shirt to prevent bleeding to the back of the shirt.
Another idea is to have your child use fabric crayons. This way, your child draws directly onto a piece of white paper. Laying the t-shirt on an ironing board or flat surface, and placing the finished drawing face-down on top of the front of the t-shirt, you would use a hot iron to press the picture into the fabric.
Note: Be careful. If you use this method, and the picture includes words or numbers, the drawing will be backwards from the original.