A good question.
A local answer.

Book a ferry that leaves enough margin before your fixed activity and check the operator's terms for return changes. Do not treat a booked return sailing as flexible unless that operator confirms it; popular dates can sell out.

If you plan to use a taxi, bus or other passenger vehicle, check the separate Transport WA rules and ask the operator what restraint can be provided. Do not assume the rental-car rule or a forum anecdote describes every vehicle type.

If Rottnest is a priority, check the ferry departure points and sailing times before deciding on accommodation. Build the day around the ferry you can actually book; your preferred beach, cafe or city itinerary can then guide the remaining nights.

Treat Wadjemup / Rottnest Island as a full outing if you want to cycle and stop at beaches. Compare the ferry port and timetable with your accommodation, choose a bike or island bus plan that matches your energy, and check weather before locking in the day.