As a landmark of Hiroshima, you really have to visit Hiroshima Castle if you're in town. The original castle was destroyed by the atomic bomb, so what you see now is a reconstruction. Some of the modern facilities inside make it feel a bit different from the original Hiroshima Castle vibe.
Besides being a great place to work out, the observation deck on the 10th floor offers a relaxing view of Kawasaki's nightscape. It's a fantastic facility.