When visiting the Blue Mountains, I recommend to stay over night around there.
You will miss a lot if you take a day trip… I think at night and in the morning is the highlight of the place.
At night, the Three Sisters (the three tall rocks on the most left of the photos below) are lit up and the sky is filled with uncountable number of stars…
In the morning, having a walk in the beautiful sunrise is very refreshing and needless to say… beautiful.
Getting to the Blue Mountains is very easy. Just take a train from the Central Station in Sydney to Katoomba Station. It will take two hours or some.