Okayama is closely associated with Momotaro, one of Japan's greatest folktales. So make sure to hit up the Momotaro festival, if you're in Okayama at the right time.
Okayama Castle is a must-see, because it doesn't look like other Japanese castles (black all around, sometimes called Crow Castle). The Komori-zuka Burial Mound dates back to the sixth century and you can go inside for free. The Naked Man Festival happens every February in Okayama. It's your chance to be (or see) a naked man.
Okayama is great. Eat kibi-dango and go Seagulls!