As a welcome sign to friends (the only one's who come in to our back door, in the kitchen) I painted this on the window.
First I drew a circle on the outside window, with a little help from a plate, so I had an idea where to paint on the inside window to get a nice wreath-like drawing. Then I started writing letters and drawing stars, snow flakes and holly leaves on the inside window. I drew something off the cuff, but you could also use a printable, stick it to the outside window and trace it on the inside window.
Very simple project with nice result!
I found this idea here.
I also did a window with X-mas ornaments.
And some transparant glass tree ornaments.