Were you planning on flying in from somewhere to SEATAC or could you come to Seattle on Amtrak. Either way you could take the Amtrak train or bus from Seattle to Vancouver. Much of each city is walkable or you can use city buses.
As long as your passports are current, the border is no problem.

We too are more city people, although we do love to hike (we walked up a ski mountain just this morning). I'd spend at least two days in each of Seattle and Vancouver. They're both really charming cities - not urban like London or New York, but definitely worth visiting.

We would be flying into and out of Seattle. Would you rent a car?

Sorry I can't really say. We flew to Seattle and took the train to Vancouver but it was just a 4-day trip, no time for anything "in between".

Thanks!

Maggie - where do you live??? As that would help us to tailor ideas to contrast what you have and know near home.

IF you've never been to this area, it is a strong probability that you'd have a great time, and if you have 6 days, I'd suggest 3 each in Seattle and Vancouver... with Vancouver, naturally, sandwiched in the middle.

Maybe rent a vehicle for 4-ish days... for the drive to Vancouver, and to strategically make your way around to outlying tourist attractions in/near Seattle (before turning in the rental car).

Stay in a downtown hotel and trust the convenient Link Light Rail to get you to the airport when it's time to leave.

We live in OK but will most likely travel to Las Vegas 9/9 thru9/11 to begin the birthday celebration.

Well, all right, so mountains and water would offer contrast to your surroundings at home.

For much of a Vancouver experience one doesn't need a car, and IF she were to weigh the added expense against the small lesser convenience of not having one... and instead taking a train to Vancouver, then that would be OK. (don't forget to include (sometimes extreme) overnight parking costs at a hotel when calculating/estimating car costs)

(gas is right NOW $3.73/GAL in Seattle... lowest I've seen, and I saw that just today)

You could take a ferry ride across the water from Seattle, for a nominal fee, and you could go just a few miles outside of central Vancouver to the foot of a mountain, and then ride a gondola car up the mountain to the often-wintery-seeming top, to get some sense of mountains while you're in this area.

(others take a day trip to Mount Rainier, from Seattle, with a one-day car rental)

I'd guess that central Vancouver would offer plenty of subtle contrast to OK... and that you could easily fill 3-ish days roaming around the downtown area and nearby surrounds. The cultural melting pot and little hints of "foreignness" make for quite the fun time. Stanley Park, the Aquarium, Chinatown, Gastown, Granville Island, and shopping on Robson Street, are just some of the attractions to be seen.