A good question.
A local answer.

Confirm disembarkation and airport check-in times, then choose whether to spend the day mostly in Fremantle or split it with Perth. Both can be reached without a rental car, but transfers consume time.

Check ferry, bike hire and weather details before committing to Rottnest. A windy or wet day can make a short local walk more rewarding than an expensive island outing.

Choose one direction beyond Perth. Those destinations lie on very different routes, and trying to connect them all would turn five days into a driving exercise.

Look for a closer coastal experience if this is your first Perth visit. Weather, daylight and rest stops matter more than the headline distance.

Consider the Perth Hills or Swan Valley for a short stay with walks and food without a long drive. Choose accommodation near the specific activities you want, as the areas are spread out.

Give one day to the city and river: choose a cultural stop, a walk and a meal in the same part of Perth. That leaves room to explore rather than watching the clock.