If you plan to drive down to Seattle, stop at the Boeing factory in Everett on the way. Tours are free if you book in advance (they were 8 years ago when I went anyway).
Go for a ride on the False Creek ferries in Vancouver too – I imagine kids will love them – and you can combine it with a visit to Granville Island market. You should definitely go to an ice hockey match as well. Vancouver Giants are a cheaper alternative to the Canucks, and there is still a great atmosphere, although there may be the occasional fight (on the ice, not in the crowd)… worth bearing in mind if you’re taking the kids!