What should I see in Sydney if I only have two days?

Keep the first day within walking distance of the harbour instead of crossing town for every landmark. Circular Quay, the Opera House, the gardens and The Rocks fit together naturally, with plenty of places to pause for lunch. I would make just one timed booking, if any, and leave the rest of the day open. That gives you a better chance of enjoying both the view and a relaxed dinner.