I promised someone I would ask my sister for recommendations in the Vancouver area and while I got the info from my sister, I never posted it. I thought I'd post it here so it comes up in a search if anyone is planning to go. Here is here advice:
Here are the things I ABSOLUTELY recommend!
Bike tour of Vancouver (it is the best way to see the city): https://cyclevancouver.com
Day trip (or overnight trip) to Victoria - the ferry itself is spectacular, and often on that route in the summer you'll see orcas.
Hikes/walks in North Vancouver:
https://www.capbridge.com
or just visiting the North Shore Mountains.
Heading for a day to Whistler/Squamish:
https://www.whistler.com/activities/
You can do ATV tours, horseback riding, ziplines, hiking, mountain biking, and it is just a SPECTACULAR mountain.
Visiting Granville Island:
http://granvilleisland.com
they have great bakeries, restaurants, and a year-round market and craft fair, with a decent french food stand that sells real macaroons! You can also take an inlet ferry in False Creek, and either go to the other side, or on a tour of False Creek, for pretty cheap!
I would also recommend a visit to gastown, where you'll find awesome canadian gifts for those back home! (Maple candies, maple cookies that are DELISH, cute clothes/collectibles, and a cool downtown feel!)
I will say, having been to both places, and not being a fan of canada, I would still prefer to visit Vancouver. It is a GREAT city for a visit, and a beautiful area! (This is because originally OP asked about whether to visit Seattle or Vancouver.)
Hope this helps!