Head out to the Indian Ocean beaches at Cottesloe (15 mins on train from Perth) and go to the Blue Duck overlooking the beach. Time it late afternoon and sit on the outdoor terrace with a mango daiquiri and something simple to eat from the extensive menu (the potato wedges are great!). If you've timed it right you'll be able to watch a stunning sunset over miles of glorious beaches. A second mango daiquiri will prove to you life doesn't get much better than this!
The Blue Duck
Cotteslo Beach
Train from Perth to Cotteslo (also a bus but not sure of the number - train is easiest I think)
www.blueduck.com.au
Google map: tinyurl.com/nqe2vn
Updates
Cottesloe is certainly a nice spot, and I agree about the beach and the sunsets, but the Blue Duck is a bit over-priced for unremakable food.
Is this your business?