From Seattle to Alaska, you should pass by Vancouver, B.C., Canada. Well worth visiting.
One route north from Vancouver is the Sea-to-Sky Highway from Vancouver to Whistler. This is a much better route than going thru the Fraser Canyon for heading north.
Sea to Sky Highway Reviews - Vancouver, British Columbia Attractions - TripAdvisor
Then from Whistler to Permberton-Duffy Lake-Cache Creek. The following webpage is dated, the road has been repaved. I drove it this past October.
Duffy Lake Loop
From Cache Creek, you can go to Kamploops and head north towards the Alaska Hwy.
Depending on when you get into Vancouver, there might be track days or autocross events locally. Usually there is almost an event every second weekend during spring and summer. I am the founder of a very active sports car club in Vancouver. If you want to attend any of our events, I will get the club to waive the entry fee. For more info on Vancouver events, post questions on their forum here, VCMC Motorsport Club Forums - Sponsored by PDM Racing - Latest
May 19-20 is the infamous Knox Mountain Hillclimb event in Kelowna, B.C.. If it fits your schedule, I highly recommend you drop by to check it out. It is the longest running paved hillclimb event in North America. An "OLD" video on the event,
More current videos on the event, knox mountain hill climb - YouTube
Great roads leading in and out of the interior part of B.C. Quite a few drivers from my club will be there. The king of the hill who holds the all time fastest record just turned 71 years old this month, he is a good friend of mine and will be attending the event for the last time this year. KNOX MOUNTAIN HILLCLIMB
If you need a place to stay in Vancouver, My wife and I will be glad to host you. My daughter has moved to Ottawa and her room is available. I have a Caterham 7 and could accompany you for part of the route up north if our schedules work together. If you need more info for areas around Vancouver, just ask.
One more thing, when you leave Anacortes after visiting your relatives, you should take Chuckanut Drive from Anacortes to Bellingham on your way up to Canada.
Two of my sisters and their families have been living in Australia since the mid-'60s. Wife and I were there three Christmas ago visiting. Once I retired in a few more years, I would like to do a long driving trip there to explore the beautiful country of yours.
One more thing. You might find the following site helpful, it has pretty well most of the motorsport events in North America listed. Most clubs use it for registration. More and more events will get added as season begins in the northern part of the conuntry. http://www.motorsportreg.com/index.cfm/event/calendar