I think April is a great time to be in Ushuaia. I went there this year mid-April, and had also gone once before in December several years go, but by far I enjoyed the trip in April much more. The leaves are starting to change colors and it is just beautiful. The tourist season is officially over so the city is much calmer, but many of the kiosks selling crafts, souveniers etc. have closed shop by then. Some things to take note of - it gets dark rather early, around 6:30 to 7 pm so plan your activities accordingly. Also, if you were planning to go to see the penguins at Estancia Harberton, they are already starting to leave by April, so the main companies offering this excursion take it on a day-by-day basis, so you'll have to keep checking on that. As for weather, it tends to be drizzly, but can also be warm throughout the day, so the best bet is to pack layers. Right after I left it started to snow, so I think you should be prepared for that too. The National Park is gorgeous for hiking and you could easily spend a couple of days going through the trails there. I'd also recommend the Ojo del Albino glacier on Laguna Esmeralda and the Martial Glacier for hiking. Happy planning!