Katajanokka is an area adjacent to the central district of Helsinki. It's my favorite part of Helsinki, having magnificent buildings and being in its western part. The Uspenski Cathedral is one of my favorite places in Helsinki as well and one of the star tourist attractions. If you climb to the viewpoint of the cathedral there are some very good city and harbor views. The southern part of this district is dominated by the port and large ships departing or cruises to Tallinn, Stockholm and Marieenhamina (the capital of Åland islands set midway between Sweden and Finland). The rest of the district comprises housing cooperatives and some small parks like the one in front of Uspenski. If you are thinking of going to visit Helsinki and want to do it cheaply you can stay in the Eurohostel which is in this neighborhood. If you go two people can have a room for about 40 euros, there is a kitchen on each floor and a very clean and comfortable bathroom with a shower. It is very, very close to the center, about 15 minutes walk but there are also passing trams and buses, because when it's cold the wind has quite a chilly wind (this is normal throughout Helsinki but also as you are near the port). So I sometimes took public transportation.
You can have a great time in this area which mixes countryside and coastal areas. An amalgam of park, beach, boat dock, from area restaurants, including an upscale residential area, as far as I could tell from the buildings. It is located south of Helsinki, not far from the centre, and is also an area of embassies and consulates. Nearby is Kaivopuisto Park, famous and popular among the citizens as a place for concerts and other cultural events in summer.