Shinjuku Gyoen National Garden is a large park completed in 1906. Most of the garden was destroyed by air raids in 1945, during the later stages of World War II. After the war the garden was rebuilt and it blends three distinct styles: English Landscape, French Formal and Traditional Japanese.
I enjoyed the Japanese part the most - such a peaceful oasis in the middle of busiest part of the city - Shinjuku. I can only imagine how much more beautiful it looks during cherry blossom season or in autumn.
Surprisingly, you actually had to purchase a ticket to enter the park though electronic ticket gates like in train station, but hey, it was less than 2 euros.
Ticket also included a visit to amazing tropical greenhouse built in the 1950's. Horticulture work has been going on in the greenhouses in the garden since 1892.