I used to live just out of Mandurah so I know the area well.
There are several attractions including a zoo, there used to be a miniature village which was fabulous; a chocolate shop; Pinjarra and the suspension bridge; the Marina and development/shops there; the foreshore; boat rides either for lunch or a ride down the Canals; wineries, a Fun Park, nice flat beaches; and several small quaint towns not too far away.
I have probably missed a few -oh, they have the Annual Crab festival as well as markets each weekend, and fruit markets, as well as crabbing, fishing.