I am a US citizen and have a long connection (23 hours)in Helsinki on my way back to New York from Paris. Do I need a visa? Can I leave the airport and stay overnight in Helsinki?
As a US citizen, you do not need a visa to visit Finland, or anywhere else in the Schengen area for that matter, so you're welcome to leave the airport and stay overnight in Helsinki.
From the Finnish Ministry of Foreign Affairs: https://um.fi/documents/-/asset_publisher/caSu4obBG5lW/content/yhdysvallat-matkustusasiakirjat-suomeen/385107?p_p_auth=TjAOgnNm&curAsset=0&stId=47307