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I like to get out of the city.
List of answers (3 answers)
Enoshima and/or Kamakura is my first pick. Enoshima has the beach, an aquarium, the fun little island to explore. Kamakura has temples and shrines and lots of local food to try.
Hakone is fun, but most people stay there overnight. I've done day trips where I've gone to museums and the lake during the day, went to an onsen, ate dinner and then headed home.