A good question.
A local answer.

Allow a little extra time around the terminal, then focus on the ferry ride and Manly day itself.

Book at a time that leaves the child comfortable and confirm the final price and cancellation terms.

Choose the hotel whose walking routes and evening transport suit the teenagers as well as the lounge benefits.

Keep valuables and travel documents with you and allow time to retrieve every bag before leaving for the airport.

Choose a base with an easy evening journey from the Opera House, then use the other day for a compact harbour outing.

Put one major outing and a relaxed meal on each day rather than crossing the city repeatedly.

Give one day to the harbour and central sights and another to a neighbourhood or coast that interests you.

Book near your other CBD plans and leave a little buffer; availability matters more than a long list of old recommendations.

The marked taxi rank is a practical option; compare its current fare guidance with a prebooked car if certainty matters more.

Check the exact apartment category's bedding options before booking; a property's general photo does not confirm your room.

Check how the rental company bills tolls and administration fees before collecting the car.

Read recent reviews for the Sydney pickup branch, then confirm roadside support and kilometre limits.