Ed... a great great itinerary. First, you should be sure to spend time in Rio... if that is your departure or end point... it is magnificent (my favorite city in the World). If it is just a stop, you should be sure to take the ship's tour... there is too much to see and Rio is quite a large and spread out City. Either way, you're in for a treat!
BA is fascinating. It is almost a European city in many respects. Here too I recommend a panoramic city tour from the ship in the morning, then go off on your own in the pm. If you like beef, you should be sure to have lunch at one of BA's great steak restaurants... they are wonderful. If you stop in Montevideo Uruguay, be sure to visit some of the leather shops... super buys on great leather items.
Ushuaia is remote and interesting. A tour will probably be best there.
You'll be sailing through some of the most scenic areas on earth, particularly the Chilean Fjords. Don't miss a minute of it.
Again, I don't know which way you are going, but I assume you are starting or ending in Valparaiso... the port city for Santiago, Chile. Santiago is a delightful city, surrounded by the Andes mountains. The people are very friendly and it is overall a lovely place to visit. It your tour ends in Valparaiso, your flight back to the U.S. will not leave till very late in the evening. Be sure to take the tour that Celeb offers for the afternoon. Otherwise, you'll have a very boring day waiting for your flight. If you got your air from Celeb, they will provide a day room at a local hotel for you.
With the exception of Rio, BA & Santiago, the cities in the southern half of South American have less tourist infrastructure than you will find elsewhere, so, as a general rule you are better off with the ships tours.
You are in for a great cruise experience in an exciting part of the world on a great vessel. Let us know how it all goes when you return.