I think it all depends on how far you want to spend traveling. without knowing Perth that well I can give you no clear ideas. I find a good way to do it is to draw an imaginay circle around where your from, measuring say 200km and then pick a ramdom place that looks like it suites your requirements. ie near the beach, bush, forest, river totaly isolated or near civilization
The south west is very nice, although could be busy over the long weekend. where ever you go I hope you have a blast.