I've been to most of these by train but they should all be possible and the remote ones definitely easier by car. There's a lot in the area but most of these are smaller places where you'd probably only spend a few hours (each).
Bit chu Matsuyama castle is an original castle and Takahashi is a nice small town to walk around in (edit: b i t c h u or Google 備中松山城)
Maki-do and Ikura-do caves in Niimi are rather impressive (unless you are claustrophobic!). They are quite remote and the area is beautiful.
Tsuyama has a small but good
railroad museum and a large
castle park as well.
The
Bizen pottery museum and
Osafune Sword museum are interesting museums in Bizen, to the east of Okayama.
There's a unique
horseshoe crab museum in Kasaoka at the far west of the prefecture.
Just across the border in Hiroshima prefecture, Fukuyama and Onomichi are both nice cities; Fukuyama has a few good museums and a castle (all close together), and Onomichi has temples and sites in the hills right above the city as well as good views of the inland sea.
And closer in to Okayama there's
Kurashiki with museums and old-style town (it can be very crowded though and taking the train is probably easier), as well as many small shrines and temples in the
Kibi plain.
I've traveled around Okayama prefecture quite a bit and if you're interested I can give more details or upload some pictures (or feel free to PM me).