Hi! We'll be on Helsinki for just one day. If you hace to choose, which one and why! We are a couple on our early forties and love to walk around, explore, watch nature....
I didn't go to Seurasaari when I was in Helsinki, but it looks like the kind of open-air museum that is similar to Skansen in Stockholm, with people dressed in period outfits and recreating the feel of an earlier time.
Suomenlinna isn't a museum in as formal a sense, though there are historical sites and you can take a guided walking tour (which I did but then dropped out of because I was happier just wandering around on my own).
Personally I enjoyed visiting Suomenlinna and wouldn't be interested in Seurasaari. But other people would probably give you different advice. I say read the web sites and look at pictures on line, and decide which looks and sounds more interesting to you.
Suomenlinna! (It is not in Taillin though.) This was one of the highlights of our trip, and absolutely meets your requirements of walking, exploring and watching nature. However, it would be weather dependent. I think it would be miserable if it were cold and raining. It was sunny, windy and cool the day we were there and it was perfect. There are places to sit, eat and/or shop. Perhaps you can wait and see what the weather is like the day you are there.
I visited both last August. Are you visiting on a weekday or a weekend? I'd say Suomenlinna if you visiti on a weekday. On the weekend, I'd go with whichever appeals to you more.
I visited Seurasaari on a Sunday and it happened to be a day when they were having some special folk music activities. There were not a lot of people there and not all the buildings were open. It is a lot smaller than Skansen. the interpreters at the various buildings were happy to explain the history to us.
I enjoyed Seurasaari more than Suomenlinna simply because I am more interested in cultural history than military history; however, there was a lot more to see at Suomenlinna and it was a lot more popular than Seurasaari so more was open there.
I generally never miss a chance to see an ethnographic museum of any kind, though I did not visit Seurasaari when I made a day trip by ferry from Tallinn to Helsinki. I focused on their beautiful downtown for the architecture and the Kauppatori market, from there taking the small boat to Suomenlinna, which I thoroughly enjoyed exploring. It was mid-September so tourist season was winding down, no crowds to deal with.
We just returned from Helsinki, spent 9 days there. Suomenlinna if you go, pack a picnic lunch of some kind. Many spots to sit and enjoy it. You could easily do this and enjoy Helsinki in the late afternoon and evening as well.
We did not go to Seurasaari.