Christmas?? What's christmas?
I just have to put in my 2cents... I liked salzberg, and maybe it's different over the holidays, but I'm not a big enough fan that I would go there for a weekend or holiday. Be aware, we and most other guests we've taken to Salzberg find it a very sleepy town. If you and your girlfriend are the type that like to go out to dinner around 8ish, have a few cocktails at a cute or fun bar after, etc.- this isnt the town.
With the xmas market, maybe you can add a couple extra hours, but we only do it as a day trip with out-of-towners and most are done 'seeing the sites' within 3 hours. I know loads of others that love Salzberg though- mozart fans, someone above mentioned concerts, etc. So this is JUST my opinion.
Besides, if you're going there with your girlfriend, maybe evenings out/nightlife isnt what you're up for and you'd like a quiet overnight in a pretty town with a good xmas market. The best 'out and about' place you'll find there though, will likely be your hotel bar. And really, I dont recommend you stay longer than 1 night.
This is really quirky, and I haven't done it myself yet, but if you both are from the NW and like winter-outdoor activities, why not do something totally novel in the alps:
Stay overnight in an Igloo. I know, a bit crazy, but if adventure is your thing, could be fun! And she certainly wont find this opportunity in the cascades.
We've been to Rothenburg and Nurnberg, and both were pretty and worth visiting. Nurnberg had almost the exact same xmas market as the one in
Marienplatz though (but if you've never been to an xmas market, then they're all fun). Wish I had better ideas, we always go for warm holidays. If she's coming to visit, hanging out with you is what will be most important to her.
Good luck!