Half Day Trip From Rockingham WA

Not necessarily related to 4wdriving but here are some nice half days outings:

Penguin Island.
Mandurah (new marina for lunch).
Preston Beach (drive on the beach, picnic lunch, 45 mins sth but that's OK).
Do the loop of Peel/Harvey Estuary is a nice drive.
Ravenswood Hotel for a countery on the river's edge.
Dwellingup.
Tourist Boat tour of Mandurah will be an eye opener if you haven't been here for a long time.

Jarrahdale and Serpentine Dam is a nice spot to visit.

Perthburts,
All depends on what you want to do? Do you want to go offroad or stay around town. As Gone Bush has mentioned some good "onroad activities". If you are keen to get off road I can suggest Dwellingup. I have been there heaps of times. But I would take a day if you want to have a good look around. Or you could head off early have lunch there and come back. You can get maps from the rangers hut there but there is a better book that I rekon all West Ozzies should have, Its called Roads and Tracks of WA. Get the spiral bound one if available.
Matt