What to do will entirely depend on what you like to do. IMO food and shopping is everywhere, you don't need to go to any particular city to do that.
I haven't been to Seattle yet, but Boeing Factory, hire a car and drive down to Mt St Helens (it exploded in 1980 and still looks amazing - mountain face is down one road, and lava tubes and petrified forest walk down the other).
I wasn't overly impressed with LA the first time (Disneyland is boring for adults - wish I knew that before going, try one of the other theme parks if you want to do that). If you like all the Hollywood scene, you can explore the Kodac Theatre, walk of fame etc. On my 2nd trip to LA, I hired a bike and rode kms along the beaches and stopped for a spell underneath the LA airport flight path watching the planes. The tar pits were really disappointing.
Vegas is full of interesting experiences if you're prepared to do some research. There are all the shows, each casino on the strip is themed, so checking out all the unique things to do in them is fun (roller coaster, gondola rides, pirate shows, dancing fountains, the list goes on), Wanna fire a 50 calibre machine gun? wanna do a zero G flight? research, research, research. IMO don't waste your time and money doing the Grand Canyon helicopter. The flight was interesting, down the strip, over hover dam and some cool scenery, but the canyon wasn't so grand where they take you. If you want to do the Grand Canyon, go to Grand Canyon Village, which is a fair drive from Vegas and really an overnight there. Vegas is also a starting point for tours of the national parks of the mid-west (these are amazing and well worth it if you are into natural wonders, with my highlight being Yellowstone NP (sort of like New Zealands Rotorua, but bigger, better and doesn't smell), it is well worth a visit - unfortunately it is probably snowed in at that time of year. You'll need longer than 3 days for this also)
In San Francisco, most things are down at the waterfront, so take a trolley in that direction, and grab some clam chowder
for lunch
(served in a cob loaf), see the sea-lions, watch out for the street entertainers who hide behind things and scare unsuspecting passers-by to the delight of onlookers. Alcatraz (research if you need to book this). Take a trip over the Golden Gate Bridge in an old fashioned fire truck. You can do Yosemite NP from San Francisco, but be warned its a looooong day trip, 12-13 hours)
I'm sure you could easily spent 7 nights in NY, there are museums and art galleries (my highlights would be the Intrepid Aircraft Carrier Museum and Museum of Natural History). If you want to do Statue of Liberty, book last year, Ellis Island, take a horse drawn cart through Central Park in the evening (cross your fingers the horse hasn't been fed some beefarino, if you remember that from Seinfeld), Twin towers memorial, Empire State Building, Broadway shows, the list can go on forever.
But the East Coast has some other great locations also, Washington is well worth a visit. There's all the Smithsonian's and all the war memorials. Philadelphia has the liberty bell and original signing location of the American constitution. What about Niagara falls?
At the end of the day, you can't do everything in the short amount of time you have available. You need to decide on the type of things you want to do and where they are. Research, research, research. Can you buy clothes in Australia? Yes, so why waste time shopping, unless its something unique that you chance upon. Sometimes it good to have a reason to go to a destination. I'd like to go back to USA to go to a monster truck show, see the American Ninja Warrior finals, see an airshow.
No matter what you decide to do, you'll have a great time.
