Ushuaia is the southernmost city in the world, it hosted the first same sex marriage in Argentina and it hasn't got a lot of population but the town is prepared for tourism and receive thousands every summer.
We arrived in Ushuaia early in the morning and it took us a long time to find an ATM that would work with our card, but once we did, we took a "combi" (a 17 seat bus) to the national park. The return on the combi costs 300 pesos each and the entry to the national park 170. You can camp in the national park. We stayed in an organised camping for 200 pesos the two of us and you can use toilets and barbecue facilities but you have the option of free camping, there are three areas of free camping and the bus will leave you right at the entrance of these places.
The national park is famous for trekking and it has 4 different trails. All except one of the trails are moderate difficulty, but we recommend that you take appropiate gear and a warm coat, the average temperature in summer is 9 degrees.
We did parts of two trails, walked for about four and a half hours. We did part of the cerro guanaco trail which is the difficult one, and we don't recommend you do this unless you are fit enough and you're fine with walking a really steep trail (the reason why I didn't want to finish it).
Allow two or three days depending on your fitness to do the whole national park by foot. More if you are not happy about walking several hours. Take food and drink from the town centre, because in the park every option is expensive.
If you camp be aware there are animals in the park. During the night we were awaken by wild horses galloping near our tent, and in the morning by falcons. You will also see foxes that don't mind being near humans, but don't feed them! It's in the rules of the park.