Yashima is a good side trip from Takamatsu in Kagawa Prefecture. There are a number of attractions on the summit including observation decks overlooking Takamatsu city and the Seto Inland Sea, the Yashima Temple, the Yashima no Ki castle ruins, and the New Yashima Aquarium.
Okayama Castle, often called "Ujo" (Crow Castle) due to its black exterior was previously a very large complex, but was however mostly demolished during the Meiji period and destroyed during the WWII air raids. Today, most structures we see today including the castle tower is a reconstruction while the only original structure is the Tsukimi Yagura (Moon-Viewing Turret). Located just across the river from Korakuen, this castle makes a perfect combined visit if you are visiting Okayama.
Sumoto Castle was a vital strategy for Toyotomi Hideyoshi's control over Osaka Bay, situated in Awajishima which is located in the strategic midpoint between the mainland and Shikoku Island. The tenshu seen today is the oldest reconstructed castle keep in Japan, built in 1928.
Odawara Castle, once stronghold of Odawara-Hojo clan, one of the most powerful clans in the Kanto region during the Sengoku period, was subjected to multiple damages due to earthquakes and demolition during the Meiji Restoration. The structures here today were reconstructed over the years, with the 3 levels, 5 stories donjon being recently renovated in 2016. Located in close proximity to Hakone, Odawara Castle makes a good day trip from Tokyo or a combined visit with Hakone.
Often overlooked castle in Nagano prefecture, Takashima Castle proves to be a little special due to its brown exterior. Though the donjon, yagura and castle gates are reconstructions, a visit to this castle is recommended if you are visiting the Lake Suwa area.
Exploring the ruins of Kofu Castle in the Maizuru Castle Park and stumbling upon unexpectedly pretty hydrangeas.
Though relatively newly reconstructed structures and inaccessible buildings (unlike favored Nijo Castle), the gardens at the Kyoto Gosho makes this a worthy visit thanks to its otherworldly views.
An unexpected enjoyable visit to Hikone Castle, one of the 12 original castles. Though the castle keep is small in size, especially in comparison to the likes of Himeji Castle, this hilltop castle offers a view of Hikone and features a number of intact buildings thus allowing visitors a more "complete" experience of a Japanese feudal castle.
Visiting the White Heron Castle, Himeji-jō, the largest castle in Japan and one of the 12 original castles of Japan.
A short trip to Hamamatsu in Shizuoka prefecture, visiting Hamamatsu Castle where Ieyasu spent 17 years during his younger days and Kanzanji, a Zen-Buddhist temple on Mt. Tateyama.
Though having lost its castle keep, Nijo Castle is a worthy visit for its Ninomaru Palace, a rare surviving example of Japan's castle palace.