The National Pass Walk runs along a cliff near Wentworth Falls in the Blue Mountains and is one of the best one-day walks I've ever done around Sydney.
The highlight of the National Pass is the series of stunning waterfalls which you pass by, through, and over. It also features far-reaching views, beautiful forest, and a great little restaurant/cafe at the end to help you recover.
Here are a few pictures, though they in no way do justice to the real thing:
Here is a rough map of the National Pass showing it running along the cliff.
I prefer to start at Burrell Drive as your stop at the Conservation Hut Cafe then breaks up the walk.
There is oodles of parking at Burrell Drive. You could also walk from the Wentworth Falls train station, however this would add another 2km either way.
Once at Burrell Drive follow the many signs to the start of the National Pass, then you are on your way. Be sure to take the National Pass and not the Wentworth Pass which runs in parallel but lower down the cliff.
The walk should take you roughly 4 hours. If you start at 10am, you should reach the Conservation Hut by 2pm for a late lunch. Then take the Shortcut Track back to Burrell Drive, an easy walk of about 30 minutes.
There are quite a few stairs involved. Nothing scary but you should be reasonably fit. Be sure to take sturdy shoes, something to eat, water, and don't forget your camera.
This walk is also a good one to do after a really good rain, as the waterfalls are then in full flight.
Totally agree; best walk for families in the Sydney area - by a margin. It catches the full diversity of the flora and fauna, and keeps the kids interested with a never ending series of ups, downs, waterfalls, and precipes. You'll never need to walk in the Blue mountains again! You don't need to be fit, just take your time. As an adult you can do this walk in under 3 hours easily; with 5-10 yr olds allow 4-51/2.