The center of Cochin is mostly like any other city in India I visited, but there are nice things to see around the city. It is still very dirty but you can find fresh fishes!
I also went to Jew street where you can find a lot of spices, it is wonderful to go there just for the smell. It is not very big but Cochin is well known for it's pepper so it is a way to discover the local culture.
Also, the local food is very good and colorful, and it was the first time I could eat something "spicy" without having my mouth burning for hours!
Cochin is famous for being a main city of Portuguese colonisation. Today, Cochin is a peacefull place which attracks loads of tourists. You have 2 solutions for your hostel: whether take it in Fort Cochin (the ancient and touristic place within Cochin) or take it in Ernakulam, the economic town close to Cochin.
In case you take your hostel in Ernakulam, the best is to take it close to the boat jetty.