The Blue Mountains are always lovely. Not difficult by train from Central to Katoomba (the main village up there) or Leura (just before).
As others say, there IS a HoHo bus, but it DOES NOT run very frequently.
Take a careful look at the timetable.
(I'd call it a HoHum bus).
Some stops are really only good for a 5 to 10 minute look around, and then you waiiiiiit for the next bus to trundle along.
If money isn't the main concern, there are some very good day-tours from Sydney on small buses. Much better than the tour on large coaches. These tours are quite a long day, but they do show you a lot.