Shukkei-en is a lovely Japanese garden in Hiroshima. Since it requires a lot of careful maintenance in order to remain an "oasis" in the city, an entry fee is charged (I can't recall if it was 2 or 3 Euros), but I think it was worth it. While it does not offer anything spectacular compared with other gardens (like in Kyoto, for instance), it isn't as swarmed with tourists because it is not as well known. Even in high tourist season, it's an ideal place to relax. Although it is especially beautiful in the fall (as you can see in one of the photos), I also have a video recorded in summertime so you can get an idea.
This is the perfect park to go to after spending the morning visiting the museums and the Peace Memorial Park. Here at Park Hijiyama tourists barely come. It's about 2 km from the museum, but you'll be there after a half-hour walk. It's a good place to unwind, perfect for walking, reading, and there are several things to see: on the one hand there's the Taman In Temple, with its buildings, statues and in the middle with a cemetery, and at the top of the park is the museum of contemporary art and a manga library for those interested in culture.