The Hunter Valley covers a huge area and is well worth a few days. The wine tours are brilliant, many to choose from, we did a full day that included lunch and 6 wineries. These are not snobby at all and the tastings are ample! You probably wont be doing much that evening. We stayed in the Crowne Plaza, a great choice. The Hunter Valley Gardens are worth visiting. Cant help with the trains, we rented a car in Sydney and drove to Cairns, all quite straightforward.

Why not consider staying in either region for a night or two. My wife and I stayed at Bilpin Springs Lodge in the Blue Mountains and loved it there.

Staying overnight is not an option as we have other things arranged. I think we may just wait until we get to Sydney and see what our options are once we are there.

You can take the train from Sydney Central to the Blue Mountains, it takes about two hours. We had planned to do the trip when I visited Sydney but unfortunately the weather forcast was for heavy rain on the only day we could go, we are planning to do it next time.
Can't help you with the Hunter Valley though.

Judith