I got A a package of Crayola Window Markers and we tried them out the other day. They were really fun and A enjoyed using them. She even asked for more. The only thing is that the markers stay wet on the glass for a bit, so the drawings are easy to smudge and can get on your hands or clothes so I would recommend using a smock. Once they are dry, you can touch them and the marker stays on the glass.
Crayola also has a set of Window Crayons, but I haven't tried those out.
(I have no affiliation with Crayola...just passing along the information of the products we use.)