If you find yourself in Calais it is very likely you have just crossed the Channel, or are about to do so. If you have legal papers, then you will not have to stay for long in this small city. The attractions are very limited, with the Beffroi (high clock tower, part of the city hall) and a small museum dedicated to the historical trade of the city, lace.
If you visit a Calais hostel in the middle of the summer, the city beach with its typical wooden sheds can be a good place to spend the day. Calais also hosts one of he largest indoor malls in Europe, which is a good place to visit when it's raining. The indoor mall is a really crazy place twice a year (January and July) when the major sales (called ‘soldes’ in French and with dates fixed by the central government) happen.
While staying in a hostel in Calais make sure you try the local specialties such as mussels and fries and some of the other seafood-based dishes. For the hikers, there are some very scenic trails along the coast that start in Calais. The hostels in Calais can also be a good anchor if you want to visit the neighboring cities by train (Arras, Boulogne, Amiens, several of them having acquired the world heritage UNESCO status). The nearby cities might not have hostels but they are also very interesting and are just a quick hop by the regional train system.
There is a very limited selection of hostels in Calais, so you will not have much choice, and make sure you book in advance to avoid disappointment, and inconvenience when you get there (hotels are more costly than hostels, and camping is not an option).