In Dec I’m transiting thru the UK on my way to Thailand, hopefully I won’t have to leave the airport to enter the country.
However, I’m spending the night in Finland, may need to check visa requirements for the EU.
Tourist/business visa are not required for US citizens for a stay up to 90 days (the 90 day period begins when entering the Nordic area: Denmark, Finland, Iceland, Norway, and Sweden.)
Helsinki is kind of boring imho, but worth an afternoon-evening jaunt. The fish market and the central square are about it. Be aware that it may be terribly cold.
If you are travelling USA - UK - Helsinki - Thailand, that’s kind of weird. Why the UK-Helsinki bit?
And if you are transiting via London Heathrow be aware that that airport is huge and it can take you an hour or so from US gates to EU gates.