Just across the moat on the western side of Himeji-jō is Kōko-en, a reconstruction of the former samurai quarters of the castle. There are nine separate Edo-style gardens, two ponds, a stream, a tea arbour (¥500 for matcha powdered green tea and a Japanese sweet) and the restaurant Kassui-ken (活水軒), where you can enjoy a bentō (boxed meal) of anago (conger eel, a local speciality) while gazing over the gardens. While the garden doesn't have the subtle beauty of some of Japan's older gardens, it is well done and especially lovely in the autumn-foliage season.
Note that a joint ticket to both the Kōko-en and Himeji-jō costs only ¥560, a saving of ¥140. These can be purchased at both the entrance to Kōko-en and Himeji-jō. Last entry 30 minutes before closing.