Margaret River is known on the international scene for two reasons. The first is most likely for it's surf; it's home to the Margaret River Pro. The second is the growing reputation for the wine of the region; some of Australia's finest!
The region itself is obviously home to great beaches just like everywhere else in Western Australia. It's also home to some European-style winding roads through forest, caves, bushwalks, and some super food... including a chocolate factory, ooerr
Also, you may as well know that the place is a popular getaway spot for a lot of Perthites.. so try to avoid it during the holidays (public and school).
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.