It would be a toss-up depending on the season.
Warmer months I would go to the Margaret River Region. Tall trees, vinyards, festivals, caves, beaches, boutique accomodation, resorts - what more could you ask for?
Cooler months I would go either:
Up the coast and stop at Monkey Mia, Kalbarri and as far as Coral Bay. Coastal beauty, dolphins, whales, quad biking, lovely weather, pinnacles, window in the world. Brilliant.
Or Karrijini National Park. 5 days is a nice amount of time to take in the breathtaking natural beauty of the north-west. Inexplicable beauty, walks, swimming in natural pools, beautiful georges, amazing colours and scenery.
But you are right - there is too much to do! Perth is a great short break. Taking in the avon catchment would be a great 5 days, or the goldfields would be wonderful and very memorable too. Then you have broom :) or one of our true hidden treasures - Esperence...lucky bay is magnificent, the national park their is breathtaking, the town itself has much to offer...not sure we want to share the brilliance of that region with the world...
Many options - all stand alone as wonderful experiences - you don't need to see all WA has to offer to have a truely memorable holiday...but I bet you will be back for more after your first trip... ;)
Born and Bred in WA :)