You can get Free Items from a monastery facility called the Amiibo Gazebo. This facility is located in the garden north of the dining Hall, as shown below. You can access and use it as soon as soon as you're free to explore.
Interact with the Gazebo then scan any amiibo to get several random items in that will appear as beams of green lights around the gazebo. You only need to scan an amiibo once and you can collect items every time you explore on your day off. You’ll obtain higher-quality items for scanning Fire Emblem character amiibos. Each Fire Emblem character amiibo will also provide unique rewards.