It's very good our capital survived so well, because older houses in central Helsinki are very beautifully made. The have small details and therefore I recommend you to look in the buildings sharply. Also if you have possibility sneak in from the houses gates to see the courtyard, it's worth to do. Usually the atmosphere there is different than in the streets.
One thing to see is also houses stairways. This area is pretty difficult to get in because you need a key to get in, but if someone let's you in then you'll get to see. If the door is open (like someone is moving or something) I recommend you to sneak in straight away! Just look like you are going home. People might get suspicious if you ask them to open the door for stairway and they will think you are going to rob them. Especially if you speak English people are very aware. This is Finland, I'm sorry.
So why to stairways? Because they have very nice paintings and lightning there is interesting too. And the higher you go, you get to see surrounding houses from another view.
Good places to see old houses are in downtown Helsinki are e.g Kruunuhaka, Ullanlinna and Eira.
And uptown Helsinki Kallio.
|Eira hospital. (pic from here)|