If you reach out and grab an Artline T Shirt Marker you could while away a couple of hours. They have 2mm polyester fibre bullet tips that are tightly covered with a handy air tight cap that can be posted on the end of the barrel when in use.  As you would expect the pigment ink is waterproof & can be washed at 60 degrees C (as long as the ink has been fixed) it’s also fade proof.