Yashima is a site to see all by itself. There is a very nice hike to the top. Yashima is the site of an historic battle between the Heike and Genji clans during the Kamakura period (12th century), there is an aquarium, and a very nice museum. You can easily spend a whole day there.
The geography of Yashima is that it is the top of a flattened volcano. It’s non-active so you should are okay for your visit. The temple was not originally a Shingon temple (the Kukai sect of Buddhism) and was built in 754 and then switched in 815. The temple still holds holy relics that were made by Kukai.
The temple bell, which is visible is from the Kamakura period.
When you get to the top you will be blown away how open and beautiful the area is. Panoramic views of where the sea meets the land are everywhere. Take a few hours of the day to enjoy the area, the views, and the temples. Post-card quality photos are practically guaranteed.