Stay at the Pan Pacific (formely the Sheraton) in Perth. Book the Pacific Club Studio, Suite or Spa room. These are refurbished and on private floors. You will NOT find anything to complain about.
All other options are on public floors and are not refurbished. Prices on Fri/Sat/Sun are half to a third of weekdays.
Kings park has good views. Go to South Perth foreshore after dark for some of the best night time city views anywhere.
Food wise you will be dissapointed wherever you go. Nothing as good as in the rest of the country.
Swan Valley is only good for awesome fruit - melons, grapes, figs, etc, but that is in February. It's waste of time for wines considering what you are used to.
If you like strawberries, go 30km up on Wannerroo road and you will find the farms that supply all the strawberries you see in the supermarkets around the country. They are truly amazing when you get them freshly picked. Worth the drive. As you go up on Wannerroo road, ignore all the signs to stawberry farms. Those are touristy crap ones. Keep going, just as the road eventually narrows, on the left, you will see normal signs and the farms are just off the road. THOSE are the ones to go to.
Fremantle - nothing as good as Lygon St.
Go 5km north of the city, Lake Monger, on the fwy side. You can spend time with wild black swans.
As Ash said, bike ride/scooter ride around the Swan River doing the circle City/South Perth is very nice.
When you go down to Margaret River, go past Bunbury (nothing to see there). Stop at Busselton and go to the pier. Then drive to Dunnsborough - beautiful country side.
From Dunsborough, you need to go back to Bussell Hwy to get to Margaret River. Don't go back the way you came. Turn right at Chain Rd, go to the end, turn left and you are at Bussell Hwy. Cuts 20-30min off the trip.
First winery worth stopping at is Palandri/3 Oceans. It's just before Cowaramup. Nice for lunch. Cowaramup, just as you enter the town, on the right you will see a shop called Settlers Ridge. It's the only oragnic wine in the region.
10km down the road you get to Margaret River. Stay at Adamson's. The only clean hotel in the whole town. Book the upstairs 2 bedroom apartment.
As you go through town, after the roundabout, drive for about 2kms. On your left you have Watershed winery. On your right, exactly oposite the entry to Watershed is the road that takes you to Voyager Estate and to Leeuwin Estate. No other winery is worth the time.
The beach areas all look derelict due to the fires from last summer.
Margaret River chocolate factory has crap chocolate, especially if you like the taste of euro chocs. Margaret River cheese factory has really good cheeses.
30km away, in Yallingup, you have Cape Lodge. The resort and the restaurant is top 10 in the world regularly. Not cheap - rooms are $1k+ per night.
All this info is current as I do all the above for 5 days every fortnight.
You make something idiotproof, they'll make a better idiot.