The most important historical site in the Faroe Islands is that of Kirkjubøur. It's just a 5 minute drive from the ferry port at Gamlarætt, so if you're taking a day trip to the island of Sandoy (or I guess Hestur, although I'm not sure what you'd do there), it's a perfect place to stop and check out for 15-20 minutes or however long you wish.
Like other actual villages in the Faroes, there's not a whole ton you can really see in any particular village, and Kirkjubøur is fairly small, but still worth your while. Saint Olav's Church pictured here dates back to the 12th century, making it the oldest church still in use throughout the Faroes. Other notable buildings at the site include the ruins of Magnus Cathedral from around 1300 and the oldest still inhabited wooden house in the world, Kirkjubøargarður from the 11th century.