In Puerto Montt I would take a tour that includes a drive primarily (to Fruitila, Puerto Varas, and perhaps to the Petrohue Falls. All to see the actual falls there is a bit of a rough walk, the drive there is lovely. There is shopping very close to the ship in Puerto Montt too...the best on the trip.
In Puento Areans I would also recommend a tour that is mostly a bus ride. Avoid the city tour. There is really nothing to see in town.
In Ushuaia plan to take a boat trip to see glaciers and penquins. You can get to the boat area very close to the pier. Town is hilling and cobblestoned, and not conducive to walking for anyone with difficulties.
On nearly all tours you have an option of staying on the bus instead of walking at each stop. The tour guide can provide you specific information as can the HAL shore excursion desk.