I would definitely make sure to do the following:
1. Tour downtown and the lights and shopping at Pacific Place and Westlake Malls. There are plenty of trees, lights, carousels, and carolers, and there's also a shuttle that will take you to the Waterfront, Pike Place, and the International District.
2. Go to Snoqualmie or another pass for a day in the snow, either innertubing or just building snowmen.
3. Check out any number of fun coffee shops or restaurants. Most non-Seattleites don't get much good Asian food, or good coffee.
4. Do a tour of the city, making sure to hit Fremont, downtown, Columbia City, the Ave, and Broadway. And show off our beautiful water by going to Gasworks park!