Sydney CBD is (1) more interesting and (2) more walkable than most major city's downtown / financial district. Having worked in the very center of it for 9 months... I've got plenty of "a day in" itineraries.
Let's start with the first timers:
Barangaroo is one of the newer areas in CBD. It's also one of the best places for food - you can get anything and everything there! Where else can you find the best croissants right next to the best donuts. My suggestion - go to Short-Stop for donuts. Try the Australian Honey & Sea Salt Cruller. You can't go wrong (it even has the name "Australia" in it)! If donuts aren't your thing, then go next door to the famous Bourke Street Bakery for a croissant or pie.
Everyone comes to CBD to see two things: The Sydney Opera House and the Harbour Bridge. While you most certainly can admire both up close, why not sit down for a bit and watch the ships go by both The Squire's Landing. James Squire always has a small batch and limited release brews rotating the taps. Can’t decide which brew to try? They have beer tasting paddles available for when you can’t pick just one – enjoy four small brews for just $20. If beers aren't your way, then stop by Messina for Sydney's famous gelatos.
If you really want to splurge for some high-end modern Australian cuisine... dinner at Quay is the way to go. Not quite what you're looking for when you're on vacation... there's plenty of other options in the area. Restaurant Hubert was a favorite of my friends and I - some incredible french food in a semi-casual, semi-romantic setting. You can't go wrong! Mr. Wong has arguably one of the best peking ducks in Sydney.