Miyajima is pretty big for a little island, and there is a lot to see when you visit. Most of this post will focus on the beautiful Daisho-in Temple.
This is momiji manju. It has been skewered with a stick, battered, and deep fried. And it is delicious! You can't find it anywhere else but on Miyajima, and it's freshly made in only a few of the stores. Be sure to ask around and give it a try. And of course, bring home some packaged momiji manju as gifts.
Miyajima is easily reached by train from Hiroshima Station - you can arrive via tram or train (which is much faster), and then take the very short walk to the ferry. If you arrive in the morning, just follow the crowds. Get a good spot on the right side of the ferry, because you get a good opportunity for photos as you approach the island. Admission is charged to ride the ferry and to enter the Itsukushima Shrine grounds. Plan on spending an entire day there - and get there early - if you intend on seeing Daisho-in Temple and Itsukushima, and hiking up Mount Misen. And remember to check the tide times so you can see the torii at high tide and walk out at low tide.