We just returned Tuesday March 16 after two nights in Ushuaia. It was very nice. About 1-4 degrees C in the day.
We stayed at a Bed and Breakfast at the following link. It's on 9 de Julio near Av San Martin.
www.martinfierrobyb.com.ar
We had a suite with three beds, bath, and mini-kitchen. It was nice and convenient to the main street AV, San Martin, but don't expect the Ritz.
The owner Javier, was very accommodating and a nice guy. He will pick you at the airport if requested.
He drove us to the glacier just outside of town. It was beautiful and I guess you should do it. But if you didn't know it was a glacier you'd mistake it for pile of snow in the mountains. Be there about 9:20 and beat the crowd. The trail gets mushy quickly. We were there at 09:20 and were the 3rd and 4th people in line. The trail was already getting mushy when we came down. We walked up there in sneakers. No big deal.
Javier can lend you coats if you don't want to bring them, or just down the hill on the right from the B&B before you reach Av San Martin, is a shop that rents clothes. We used Javier's coats. We live in Rio de Janeiro and don't own coats. Just flip flops and sun screen.
Things are more expensive than in Buenos Aires. There's lots of interesting shops and things to buy. The duty free shops don't appear to be offering any deals. The restaurants are expensive. Any dish with local crab is really expensive. The good news though, the price of wine in the grocery store is about the same as Buenos Aires and Mendoza.
There are some excursion operators that offer "walk the penguin trips". We didn't have time to do that. The prison and prison train are supposed to be nice.
We flew on Aerolinas Argentinas from Buenos Aires to Ushuaia. It stopped in Trelew on the way. But, on the return we flew Ushuaia to Buenos Aires. It stopped in El Calafate. So you might want to plan Buenos Aires to Ushuaia to El Calfate to Buenos Aires.
If figure out why people go to El Calafate, please let know. 90% of the people on the plane got off there.
If you can work Mendoza in, you should. Stay in the Micro Center area. We stayed at the The Argentino hotel on the Interdependence Square. Great location. Mendoza is clean, tranquil, cheap, friendly, has over 900 wineries, etc, etc, etc. The mountains are beautiful. Check with Trout and Wine for tours. I'd drop something off the itinerary just to work in Mendoza.