Perhaps the most beautiful Japanese garden in Tokyo, Koishikawa Korakuen is most well-known for its fall colors, though it is beautiful at any time of the year, including the plum and cherry blossom seasons. The park is so popular that it becomes absolutely packed with people during peak seasons. The pictures below were taken last February, when the plum blossoms were just starting to show. Hopefully I can see all four seasons at the park this year, as it's inexpensive and easy to get to.
For more pictures and details, see my original post on the park. The garden is still undergoing renovations, though between my two prior visits I was able to see everything. The renovations should be done in time for the cherry blossom season this year.