18 hours in helsinki

If you are comfortable finding your hotel late in the evening, I'd stay downtown, personally. There is regular train service between downtown and the airport. See if you can find a hotel close to the train station. Then you can have a leisurely morning around downtown Helsinki. Walk out to the market by the waterfront. Maybe take a ferry out to Suomenlinna island. I wouldn't call it amazing, but it's something to do. When you're ready to leave town, go back to the hotel, pick up your bags, and take the train back to the airport.

If you stay at an airport hotel, you might get to sleep earlier, but the next day you'll have to train into town and back, get back to the hotel to get your bag most likely...or leave your bags in a locker at the train station, so when you are ready to go back to the train station, pick up your bags at the train station and head right to the airport from the train station. You'll spend more time the second day going back and forth on trains. Maybe an airport hotel will be cheaper. Up to you.

I would stay close to the airport and rest. For the next day, I would arrange a private tour. So you can see the sights and not be afraid to find your way back to the airport on time. When we were on our cruise that ended in Copenhagen, we also had about 6 hours to spare. Knowing myself, if I would have done it on my own, I would have worried the whole time afraid not to make the flight on time.
Maybe you can contact Katharina from Anastasia Travel. She was very knowledgeable and she can help you decide what to do. It never hurts just to ask.

I would recommend a meal at the Old Market Hall and a walk through to explore. Great souvenirs there too.

https://vanhakauppahalli.fi/history/