Okayama Castle is one of the most beautiful (reconstructed) castles in Japan, if not the most beautiful. Also known as "crow castle" due to its black exterior, it's situated right on a river leading out to the Seto Inland Sea, overlooking the beautiful Korakuen Garden.
The original castle was destroyed during World War II, and the existing structure was finished in the 1960s. Only one original building remains, but foundations of old buildings and reconstructions help give a bit of an idea of what the former castle looked like.
Some building layouts are identified by stone. There is at least one place where you can go "underground" to see the layers of trash and remains piled up on the castle grounds. Castles frequently burned down or were destroyed, and new ones would be built right on top.
The castle is modern and well-kept, so exterior decorations shine in contrast to the black surface of the keep.
The inside has a bunch of exhibits on several levels, but you can start at the top to see the view. First, there are the outlines of some structures in the area outside the castle.
Looking off in other directions, you can see Korakuen just across the river, and the pedestrian bridge between the two.
The exhibits are mostly standard, with some interactive ones. Actually, as far as modern castles go, Okayamajo is one of the best due to comprehensiveness.
In addition to the usual armor and swords, there are a bunch of traditional toys and exhibits on city life, including a pottery-making class!
Besides the exhibits, you can get a snack and pick up some standard souvenirs in the ubiquitous gift shop.
Okayama Castle is easy to get to. Walking from Okayama Station will take about 30 minutes - just walk straight east following the tram tracks, and continue heading east when the tracks turn. You can save a little time by catching that tram and getting off at Shiroshita Station, and again walking straight east for about 10-15 minutes.
The castle is open 9:00-17:30 (closed Dec. 29-31) and admission is only 300 yen. There are different combo tickets available, but you'll probably at least want the 560 yen ticket that also includes Korakuen Garden. There are some special exhibitions where tickets are 800 yen, and combo tickets aren't available.