A good question.
A local answer.

Choose a few beaches or towns that fit your interests and daylight.

A small data allowance is harmless if the total price and call conditions suit your stay.

Albany gives a larger service centre; Denmark may suit a quieter coastal stay, depending on your accommodation.

Price the long drives and accommodation before committing to a route that looks short on a map.

List the days when a car gives a real benefit, then compare rental, parking and fuel with transit costs.

Compare total cost and walking distance to the hotel, not just the fare quoted per passenger.

Keep one flexible day for weather and a longer meal in the region you enjoy most.

Check current Western Australian child-restraint rules for the exact vehicle and child's age.

Place the spare night where it breaks a drive or gives your favourite region breathing room.

Check Transperth's current payment options for each route before your first trip.

Confirm hotel transfer, check-out and the onward flight's check-in time; do not assume an old forum visa answer applies.

Allow for the transfer from your hotel and check the weather, surf and lifeguard information.