There is a lot to see on the island of Sandoy and at the end of 2023 it will open a sub sea tunnel so no more taking.
Once you get to the island you can start by driving to the village Skopun and hike up to Líraberg & Lokkloysi. These hike must be made with a guide so the easiest thing to do is contact the tourist information of Sandoy here - https://visitsandoy.fo/en/frontpage/activities/hiking/