Chidorigafuchi, Ueno Park, Yasukuni Shrine, Meguro River are beautiful. You can get on boats at Chidorigafuchi and enjoy the view from the water. Yasukuni Shrine and Ueno Park have food strolls so enjoying street food is also a fun part. There are many restaurants around Meguro River so you can sit down and relax. But you should expect lots of people.
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I have two suggestions in Tokyo. The imperial palace near from Hanzomon station, or Inogashira park near from Inogashira station. You have to keep the space early in the morning (or even the night before), in case you would like to do a typical Ohanami. But if you just would like to walk watching the cherry-blossom, you don't have to so worry about.