Bob Edwards:
My ladyfriend and I are going to go from Oakland to Whistler in a few
months for a wedding and I'm trying to decide how we should go. It
seems as if we have several options. We can fly to Vancouver via
Seattle and then take a train or rent a car and drive to Whistler or
we can fly to Seattle, rent a car and drive to Whistler from
Seattle.

Uh-huh, that sounds right. The train from Vancouver to Whistler is a
premium operation and actually leaves from North Vancouver.

My inclination would be to drive to Whistler from Seattle

Then by all means do that.

Don't forget that you'll need passports (or other "WHTI-compliant" ID --
Google it) at the border these days.

but I have no idea how long a trip that would be

Seattle to Vancouver is about 140 miles of freeway (I-5 and BC 99).
Vancouver to Whistler is about 60 miles (still on BC 99, but it's now
an ordinary road). If you avoid delays due to traffic in both cities
and also at the border crossing, you might do it in under 4 hours.

(If my experiences last time I was in Detroit are representative,
long delays at the border will be much more likely in the southbound
direction. Allow for this if you plan to drive directly from Whistler
to the airport to catch your flight home.)

or if we would encounter any problem taking a rental car out of the US.

Depends on the rental company. Major companies don't mind you going
into Canada (Mexico is another matter), but cut-rate ones might not
allow it or, I suppose, might charge extra. Tell them where you want
to go when you're making the reservation.

Of course, the primary markings on a US car's speedometer will be in
mph instead of km/h.
--
Mark Brader, Toronto "Asps. Very dangerous. You go first."
-- Raiders of the Lost Ark

My text in this article is in the public domain.