I have 3 days in Helsinki and am considering a daytrip somewhere. Looking for something with more of the typical wooden housing of the Nordic countries, to contrast with Helsinki. Some possible ideas are Parvoo or Tampere. I'm thinking of saving Tallinn for another trip, unless someone has a good reason to convince me otherwise. I know that Finland rarely shows up in this forum, but has anyone out there in Ricknik-land explored the country outside of the capital?
We only visited Helsinki. The highlights for us were Suomolina Island and the downtown area. The rock church was just okay and not really worth the trip to see. Have you considered Turku? It's the oldest city and was the main center when the Swedes were there. Helsinki didn't become the capital until much later, it started as a supply hub for the Island fortress. I do think the ferry to Tallinn is a good idea. The Baltic Capitals face the sea because it was the primary link for trade and travel in the region, which makes Helsinki and Tallinn practically next door neighbors.
I considered Turku, but from what I've read, it burned down so many times that the oldest building in town dates from the 1820s and it's now mostly a relatively uninteresting modern city.
I've heard good things about Parvoo but have not been there myself. I did do the ferry to Tallinn and it makes sense to do if you're in Helsinki. It's only 90 minutes and Tallinn can easily be seen in a day. It's a very different character than Helsinki - much more Medieval and in fact is one of the most intact Medieval cities in Europe (not having been bombed or burned). There is also some Russian imperial architecture but not as much as in Helsinki.