We did the South America cruise in 2001.
We also arrived in Santiago a few days early. We made arrangements through our hotel for a "tour" to Valpariso/Vina del Mar on the morning on the day we needed to board ship. It was a great tour to see things along the way; we had lunch at a seaside restaurant in Vina del Mar and were delivered to the ship at about 3pm. Our ship was scheduled to depart at 6pm, so we had plenty of time.
The cost of this tour back then was half of what Princess wanted just for the transfers although our lunch was not included.
You might check with your hotel and see if they can arrange a city tour for you also. I found that the hotels were very cooperative and helpful.
Any other questions, please feel free to email me.
By the way, let me know if you are stopping in Puerto Montt. I can recommend a wonderful private tour guide.