I don't usually decorate the front of my christmas card envelopes but I had these rather nice kraft envelopes which I thought needed a subtle jooshing-up. I didn't want to glitter-to-the-max but I wanted a proper wintery theme to reflect Yule and the winter Solstice as well as Christmas. These stamps from Card-io fitted the requirement perfectly and you don't need a lot of "stuff". Something sparkly, a white and a black ink pad and you're good to go.
If you decide to do something similar, don't forget to leave space in the middle for the address and another top right for the stamp! It's easy to get carried away.
Here's the video so you can see what I did and I hope this inspires you. Enjoy!