The Ritsurin Kouen is one of the most beautiful garden I ever saw in Japan, and it is huge! (Around 750 000m2)
There is a lot of ponds and hills, very beautiful trees and some pavillons in which you can drink tea and eat japanese sweets (wagashi).
The north part is in a japanese style, and the south one is more like western style gardens. I recommend this place if you want to relax yourself while admits the beauty of the garden, and enjoy a delicious refreshment.