A huge lake takes up most of the area, and there is a nice walking route around the lake. The garden also features Japanese scarecrows, with their sinister faces; great if you haven't seen a scarecrow in Japan before. Great too if you have.
The gardens also aren't usually busy, so if you want to wander around and explore alone and in complete silence, then I would highly recommend a visit to Koishikawa Korakuen Garden.