Take a short ferry to an island
In Kanagawa, next to Tokyo, there are islands you can reach with a boat ride of just a few minutes. Even a short crossing feels like a trip once the pier drops away and you are out on the water. No booking is needed for these boats, so you can decide on the day; just check the weather, because they may not run in rough seas.
An uninhabited island in Tokyo Bay, about 10 minutes by ferry from Mikasa Terminal in Yokosuka. Brick tunnels and gun batteries built to guard the bay are hidden in the forest, so you can explore them as if on an adventure. You can also have a barbecue on the beach without bringing any gear, on days when the ferry runs.
An island at the tip of the Miura Peninsula, about five minutes by small ferry from Misaki Port. Walk the coastal path to Umanose Domon, a natural rock arch, look into tide pools at low tide and visit the island's two lighthouses. In winter, flocks of cormorants rest on the cliffs and wild narcissus bloom.