I have lived in WA all my life (35years) and have seen a huge lot of it, but still not scratched the surface. I have also been to all the other states, bar N.T., which is next on the list.
We have many, many unique things that I would highly recommend. But if it came down to one thing, I would say, hit the beaches. We sandgropers tend to take our beaches for granted over here. But when you compare them to other places in Australia and the world, you don't get much better.
The East might have The Great Barrier Reef, but do they have fish come up and nibble your toes in ankle-deep water like Coral Bay? No, you have to take a 2-hour boat trip.
Our sand is often so white it's blinding, so soft it squeaks underfoot, and stretches so far in some places that you don't see another person for days.
In Esperance, the water is so aqua blue that your friends will swear the postcards have been tinted, and that you actually holidayed in the Maldives.
Then besides beaches:
The NT has a big rock, but ours is bigger, older, has more rock art, and is less trodden on.
We have living rocks, the biggest trees, the hottest desserts, the thickest forests, canoeable underground streams, the best wine, the longest highways, huge gorges, heaps of caves, friendly dolphins, gold and diamonds under our feet, mighty rivers, a walk in the clouds above the trees, incredible history, gorgeous and unique wildlife, the cleanest water and the freshest air, and the friendliest locals...just to start with.
Sorry, 5-7 days is nowhere near enough !!!
ps. Am I bias...of coarse not, lol !