We used window pens before, they are great to motivate the pupils to write sentences and the window will be a lovely display area.
It was an expensive activity until I found some very cheap white window pens in a local store last week (the Range), 4 for £1. I tried one and the quality looks OK. I also bought some coloured pens, these were £8.
The one thing I have never tried are window crayons, but now I have a beautiful set of them, too.