I'd definitely recommend a cruise along the Swan River to Fremantle, and then a couple of hours wandering around Fremantle's historic streets, cafe strip and the markets, if they're open. Nature lovers should try to do a quick trip out to Rottnest Island, as it's so unique, while Perth's cultural precinct in Northbridge (The Western Australian Museum, Art Gallery of WA and the State Library) are also well worth a visit. If time allows, watching the sun set over Cottesloe Beach after a relaxing swim is also pretty cool...