Uyuni is a small town in the middle of nowhere in the south of Bolivia. It used to be a mining centre with an active train system. So, you can visit the train cemetery at the end the town and see the many rusty steam train remains left there.
Nowadays if you make it to a hostel in Uyuni, it is because you really wanted to get there. The trains are very scarce and you have to endure several hours of dirt road in a derelict bus if you have not hired a (very expensive) four-wheel drive vehicle in La Paz.
Currently Uyuni relies almost solely on tourists to stay alive. It is the main hub to the world class Salar (the largest inland salt lake in the world, though it's dry most of the time). Numerous agencies sell trips to the lake ranging from a day to a week, at different prices. Prices are not necessary directly linked to the quality of the trip (they all go to the same place). It is advisable to browse around if you have time, ask about the different tours at your hostel in Uyuni, either the staff or other travelers should be able to help steer you in the right direction. Don’t attempt to travel inside of the Salar on your own, since you could easily get lost and wouldn’t be able to get help in case of a mechanical breakdown.
There is not much else to do when you stay in an Uyuni hostel, except to rest after your Salar trip, and share horror stories about it (you can find many bad experiences related on internet or on the notice boards of the Uyuni hostels, true or not…). Try to find a quiet Uyuni hostel. There are some hostels outside of the town itself, but without a guide you will probably not be able to locate them, or even find a bed, since they mainly work with tour agencies.