If you don't mind spending a little extra, consider staying at Northstar. You guys will really enjoy that village in the evenings. Squaw has a nice village too, but may be even more expensive than Northstar. For the type of riding you want to do, I think N* would suit you fine for one of your days, and definitely suit your girlfriend perfectly. Long cruisers and perfectly spaced trees if you get snow. You could spend most of your day on the backside and have a blast.
Squaw and Alpine are going to have more intense terrain, but it doesn't sound like you're looking to get too crazy and who knows what the base will be like in early Jan. I haven't made it to Homewood yet, but have heard nothing but great things. The terrain seems comparable to Northstar with long groomers and nice trees from what I hear. It may have some steeper options than N*. People will tell you N* is flat, and that may be true, but just stick with the backside and the Lookout Mountain/Martis area (up the tow rope) and you'll have a blast.
Make sure you rent a 4wd vehicle just in case. If you don't, and it snows, you're not driving anywhere. Really, if you stay at N*, Squaw, Tahoe City, or Truckee, you'll be within 30-45 min driving distance to any of the mountains I mentioned, with Homewood being the furthest since you actually have to drive along the West shore of the lake (which is a nice scenic drive and you get to see the house Godfather II was filmed in).
Someone mentioned staying in Incline. That's a great area, but it's gonna make your drive further. Incline is in the NE corner of the lake and the mountains you want to hit are in the NW corner (Homewood being more W than NW). However, I HIGHLY recommend taking an afternoon/evening drive to Sand Harbor in the Incline area after a day of riding. As far as easily accessible spots on the lake go, it is definitely my favorite (see pics below). It is the easiest way to truly experience the lake and it is a shame when people come to Tahoe and don't. Hope this helps, let me know if you have any more specific questions and I'll check back shortly.