I go up to Vancouver from the Seattle area at least once a year. I usually go over the Thanksgiving weekend, and then another weekend in the spring if work permits. This coming week I'll be four days in Victoria, which is my trade off city.
I drive, simply because I don't stay in the downtown area all the time. On the US side of the border the road is fairly scenic but busy, with traffic most of the way. On the Canada side it's flat and kind of boring. And the wait averages an hour plus either way on the boarder for Customs. Yes, you can get around using public transportation, but I like the flexibility a car gives me. There is no problem taking a rental into Canada, just let the company know. Parking can be a problem, and it's not cheap, but frankly downtown Seattle is more expensive.
If you're planning on seeing Vancouver then fly into Vancouver. You'll waste the equivalent of a day of your vacation traveling to and from Seattle.
Travel to and from the ski areas is problematic. You're going to have to figure in the weather and holiday crowds. If you want to ski you should have booked about 4 months ago.
Vancouver is expensive, but has great stuff to do if you aren't afraid of being outside in cool, wet, weather. Excellent parks (Stanley, etc), museums (try the First Nations one at University of BC), zoo, aquarium, art galleries, Granville Island, Asian food, seafood, shopping (the strong dollar helps), and more.
Be advised, if you have a DUI or drug record you will not be admitted into Canada. No joke, they will make you turn around. Likewise, although Marijuana is legal in both Washington State and B.C. it's use will get you denied entry, and transporting it in any amount is a Federal and National crime and will get you jail time.