In Ushusia we just walked around the cute little town and shopped, but there is a very nice history museum there (limited hours) you can walk to, and many people took boat trips here. I heard that the train excursion was vastly over-rated.
In Port Stanley I did not take a tour. The town is right at the dock and you can walk easily to all the sights here. I took the shuttle bus available at the dock ($10 round trip, credit cards accepted) to the penquin sanctuary, which included a brief drive around that end of the island. The docents at the penquin sanctuary were very helpful and led short hikes and talked about the animals, birds and plants. Even out of season it is a beautiful area, although seeing all the warning signs about land mines was a bit scary.
Took a free shuttle to a craft and leather store near downtown in Montevideo, then walked over the the large park nearby and just people-watched. Walked back to the leather store, bought a few souveniers and took the free shuttle back to the ship.
In Buenos Aires we took a city tour through www.southstartours.com
and the went back to Calle Florida by cab and just walked and shopped. Lots of internet cafes in this area too. Go into a good hotel to get a reliable cab back to the ship through the desk staff instead of hailing one on the street.