Hi Judit,
I really recommend the Kasai rinkai park (葛西臨海公園). Its amazingly beautiful, it's a civic park close to the sea and nature where you can enjoy an entire day too!
You can find more info, address, map and everything here:
https://planetyze.com/en/japan/tokyo/kasai-rinkai-park/map
And pictures, etc here:
https://planetyze.com/en/japan/tokyo/kasai-rinkai-park