One of twelve originals
Japan has only twelve surviving castle keeps that predate the modern era and were never destroyed and rebuilt in concrete — Hikone Castle is one of them, alongside Himeji. Completed in the early 17th century, its compact three-storey keep is more modest in scale than Himeji's, and precisely because of that, it draws a fraction of the crowds.
Genkyuen Garden
Adjoining the castle grounds, Genkyuen is a traditional strolling garden built around a pond with the castle keep visible in the background from several points along its paths — a quieter, more contemplative stop than the castle itself.
Combining it with Lake Biwa
Hikone sits directly on the lake's eastern shore, making it a natural extension of a Lake Biwa day trip rather than a separate outing — the JR Biwako Line continues up the shoreline from Otsu toward Hikone with no need to backtrack through Kyoto.
Getting there
Hikone Station sits on the JR Biwako Line, roughly an hour and a half from Osaka with a change, putting it at the outer edge of what's realistic as a single day trip — factor in a full day rather than trying to combine it with much else.