Beautiful scenery in Ushuaia, well worth the trip, at least in fall when I was there. As one would expect from a small town in the boonies very far from anything else, where winters can see a lot of weather, the architecture is utilitarian. The people, however, were great, and it was interesting to wander around.

I don't know how severe the weather will be, but even in April I wore a winter coat. I don't know if you'll be able to get out to the national park that time of year. A day to explore the town is sufficient if you don't have anything else planned.