Write your proposal in the snow with food coloring - just bring your partner outside by suggesting going for a walk to get hot chocolates and propose in a quiet area. You could even have the proposal written in the snow ahead of time.
Take them on a sleigh ride and propose as you snuggle together under a blanket.
Scratch it out in the ice on their wind shield to find when they comes out to their car in the morning. Be waiting at the door so that you can pop the question.
Have a snowball fight with the ring in one of the snowballs. When the snowball crashes on them, have them find the ring, or point it out right away.
Build a snowman and have it hold the ring box in his stick hand. Have your partner come look at him and propose when they see the ring.