Fremantle is Perth's port city, sitting at the mouth of the Swan River about 20 kilometres southwest of the CBD, and it is one of the best-preserved Victorian-era port towns in the world. The combination of limestone warehouse architecture, a working fishing harbour, a strong arts scene, and what is widely regarded as Australia's best beer (Little Creatures, brewed in a converted boat shed on the waterfront) makes Fremantle the kind of day trip that turns into an afternoon, then dinner, then a long drive home in the dark. Most Perth visitors make it here at least once; many end up thinking it was the best day of the trip.
The Fremantle Markets in the South Terrace precinct have been operating since 1897 and remain genuinely good: fresh produce, local food, crafts, and music. The Fremantle Prison (a World Heritage Site) runs excellent tours. Cappuccino Strip on South Terrace is the outdoor cafe scene that Fremantle has been doing since the 1980s. The WA Maritime Museum on the waterfront covers the state's seafaring history and houses Australia II, the yacht that won the America's Cup in 1983. Getting there is easy: 30 minutes by train from Perth station, or a more scenic route by ferry up the Swan River (longer but pleasant). Allow a full day.