Uni Posca Paint Markers
Posca paint pens are (in my opinion) a must have for any art supply stash! These water-based markers can draw on just about anything - rocks, glass, plastic, leather... you name it! Since they are water-based and non-toxic, they don't have a strong smell which is a bonus.
There are various color sets available but I chose this one because it also includes metallic gold and silver: