According to ancient archives, Miyajima would be ranked in the top 3 of the most scenic spots in Japan with Matsushima in Tohoku and Amanohashidate in Kyoto. This island is now registered as a World Heritage Site by the UNESCO.
This island has been an important spiritual spot since the Ancient history and started getting more developed during the 12th century when Kiyomori Taira, the most powerful lord of this era, started protecting the Itsukushima sanctuary, to eventually finish the actual building's base. Since then, the sanctuary has been through many fires and has therefore been rebuilt and enlarged.
You can access it from the Miyajima Guchi port by ferry, it takes approximately 10 minutes and you can admire a big Torii (traditionnal gate that stands at the entrance of Shinto sanctuaries) in the middle of the sea, beautifully growing as you get closer to it.
The actual Torii is crimson colored and is 16,6 meter tall for a weight of 60 tonnes. It is the 8th to be built since generations and was erected in 1875.
You can even walk to it when it is low tide.

Deers will welcome you when you arrive at the Miyajima port.
Nara's park and Miyajima's are the most famous sightseeing spots when it comes to see wild animals. Both are deers from the same breed, but their personnality and the way they are being treated are really different.
The Miyajima's deer is smaller than Nara's, but he seems to be more aggressive. Nara's deer has been venerated as one of the gods's messenger since Ancient times and is now protected as a natural monument, it became a real tourist attraction.
 
D'autre part, les circonstances sont légèrement différentes à Miyajima, le cerf à Miyajima n'est ni un messager de Dieu ni un trésor naturel.
L’île entière est censée être un lieu sacré et l’atteinte à tout être vivant est interdite, si bien qu’il est impossible de stopper leur prolifération.
Miyajima's deer conditions are quite different, it is neither considered a messenger of god nor a natural treasure.
The whole island is supposed to be sacred and hurting any living being is strictly prohibited, which makes it impossible to stop their proliferation.
 
On Miyajima Island, it is strictly prohibited to feed the deers, but if a craving deer gets close to seek for food, it is very likely it will swallow anything, even paper, by mistake. You should therefore be careful with what you carry in your bags and what you are holding in your hands. 
However, do not be afraid as deers remain harmless animals.

 
You can see the sanctuary while walking through the port and the very lively street of Omotesando. The crimson colored temple has noble architecture style called "Shindenzukuri", which connects the main hall to the other buildings through beautiful corridors.
As many buildings built in the ocean, when high tide, immerged stilts carry them, giving the impression that the sanctuary is floating on the water.
During Autumn and at the beginning of Winter, when it is high tide water might overflow in the corridors which might cause the sanctuary being closed for a few hours. Yet, despite typhoons and tornados, it is almost a miracle that none of the mains parts of the sanctuary had been majorly damaged until now.

In the temple's cave, it is said that you can receive an immediate blessing from Buddha that is worth the one received after the 88 temples of Shikoku pilgrimage. You can also see a lot of Buddhist statues as well as other interesting spots and items like Ichigan Daishi, Fudo Myoo, Jiwo, etc...

In the Momijidani park, just behind Itsukushima's sanctuary, you will find a beautiful walking path from which a cable car is heading to the top of the holy mountain "Misen". The 360° panorama view that appears from there, surrounded by a rainforest has been venerated since Ancient times. If the weather is good, you will be able to see the Seto inland sea islands. It takes about 30 minutes to climb to the top of the mountain from the cable car station, therefore this itinerary is recommended for people with good physical condition.