You should try to see Mt Ranier - it's not far from Seattle. Also, there are some beautiful islands around Seattle - you get there by car ferry - in fact, you can get to/from Vancouver from one of the car ferries (if my recollection is accurate).
Seattle - the web will tell you all the good tourist sites (Pike's market, etc.)
SanFran is a great city - you won't need a car to explore the city - but if you want to venture into the surrounding areas, the car will be handy.
You may want to check out Vancouver Island as well, which can be reached via Vancouver.
Note - crossing the border from Canada to the US is a pain - they are not friendly (and I hold a US passport) and they assume you were doing something suspicious. Make sure you have all your papers in order (passport and the new electronic visa required FOR ALL visitors - even under the US visa waiver program - as well as evidence of your flight home from the US to Australia - they always want to see that you have plans to leave). You apply for the electronic visa online and it is FREE - there are scams out there where they say they will get it for you for a fee. Don't use these - it's very simple (I've done it for DH), and doesn't cost a penny if you go to the proper website.
Have a great trip - it sounds awesome...and NY will still be there for next time.
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