Naoshima must be one of Japan’s most famous and popular small islands among foreign tourists. When I visited a few years ago, it looked to have more foreign tourists than Japanese. It’s an art-themed island in the middle of Seto Inland Sea with outdoor sculptures (lower right photo) and top-notch art museums. Pop artist Kusama Yayoi’s polka-dot pumpkins (upper left and right photos) are the island’s most famous and iconic. You can also go inside the red pumpkin (lower left photo).
It’s a rare success story for a rural, out-of-the-way Japanese destination trying to attract foreign tourists. If you want to see all the artsy attractions, you may need to stay overnight (not cheap). A day trip is enough time to only hit the major sights and museums. The popular art museums require reservations online in advance.
To get around the island, you can rent a bicycle in front of the ferry port, but be prepared for hilly terrain. Otherwise, ride the local bus or the free shuttle bus. Be sure wear a mask on the island and follow the basic safety protocols. 
Go from Uno Station/Port in Okayama Prefecture or from Takamatsu Port in Kagawa. The boat ride across the Seto Inland Sea to get to Naoshima is also scenic.
Official site: http://www.naoshima.net/en/
More photos of Naoshima: https://photoguide.jp/pix/thumbnails.php?album=919