Stargazer, we did that in reverse, plus Antartica on HAL Ryndam 4 years ago. We took a tour that featured a boat ride on a lake at the foot of two huge volcanoes near Puerto Montt. I highly reccomend that. Not much at Ushuiah, which is the end of the world. You might want to just walk around town there. There was a penguin tour of some kind though. As for Punta Arenas, it is a very interesting small city with a very nice walking tour.
You have a real treat in store with the "inside passage" (reminded me of Alaska in some ways) between Punta Arenas and Puerto Montt. spectacular scenery with gigantic glaciers nearby and fantastic scenery.
Be sure and rent and watch 'Evita' before you go and go see her grave at Recoleta Cemeterio while in BA. The cemetery is unique in the world! One of the evening tango shows is well worth the money too!
It will be a great time! bon voyage