Bunbury is a quaint coastal city 2 hours south of Perth and about and hour north of Margaret River. There's plenty of shopping, cafes, restaurants and the best aspect, the beach and harbour.
Koombana bay harbour is host to the Dolphin Discovery Centre which monitors the pod of dolphins that live in the Bay and surrounding waters. They also run interactive cruises if you want to get up close to the dolphins. Koombana bay may not be the nicest beach in Bunbury but is perfect for children and families and is packed in the summer, but just be careful of the jellyfish!
The Leschanault Estuary is a great place for all those with a love of water, with activities such as fishing, crabbing, boating, windsurfing and kayaking.
The nicer beach is the 'back beach', 15 minutes walking distance from the city. It's beautiful, some days good for surf and body boarding, other days perfect for just floating around. There's a native wildlife park - 'Big Swamp' also, to get up close to some Kangaroos and other native creatures.
Victoria Street, the main road in town, plays host to some great cafes and restaurants, such as Henry's and Mojos, whereas Dome in Marlston Hill overlooks the estuary and Koombana Bay and is the perfect location for an outside cafe lunch.
230km from Perth, the seaside town of Busselton can be attacked whole-heartedly for a day-trip.. but why bother? Stay overnight on your way further south and see what's on offer.