[Death by Mirrors]

Depending on whether you want classical cards or geeky, there's plenty of options. It's easy to spend hours on pinterest or similar sites. I think I'll be sending a fellow Game of Thrones fan one with a snowman dressed up as the night watch's most famous bastard Jon Snow and a slight twist on the original quote - "You know nothing, Jon Snowman". Been pondering to do the always symbol from Harry Potter as a christmas tree, but then again I don't know if Potter fans would perceive it as a declaration of love, and that could be awkward.

As for classical, lots of trees, stars, bells and ornaments do the trick. Different colors and textures, and you suddenly have a whole lot of variation.