The oyster (kaki 牡蠣) festival on Miyajima island is held each year in February during oyster season. It's right next to the ferry port and a no-brainer if you are into seafood. You can get many variations of oysters: grilled, fried, in soup, battered and friend in tempura ... They also sell other seafood like garden eel (anago 穴子) and other fish.
If you don't want to stand in line for a long time, plan to get there very early, even before the main festival starts. During the festival you can enjoy traditional Japanese culture. Traditional plays with flute and taiko drum music playing while the actors dance and fight in colorful costumes on the main stage. In combination with the festival, you can enjoy the famous Miyajima island sights.