Do I need a visa to visit Tallin for 1 day?
I hope that Russia didn't also take over Tallin. If it is still part of Estonia and you are an American: https://travel.state.gov/content/passports/en/country/estonia.html
TOURIST VISA REQUIRED: None
Tallin, Estonia does not require a visa for US citizens. Are you daytripping from Helsinki on the ferry?
Don't worry, Russia won't take over Tallin. Estonia is in NATO. Those others which were overtaken were not.