There are winter and Christmas illuminations in Tokyo. You can Google for Tokyo winter illuminations.
Can consider afternoon to evening/night trip to the Ashikaga Flower Park for the winter illumination.
Narita is pretty nice and enjoyable for a day trip. Three Storied Pagoda, Yuhi-no-Taki Falls, Naritasan Shinshoji Temple and Narita Omotesando. Can consider staying in Narita for 1st or last day if your flights are to/from Narita.
Likewise Mt Takao is a pretty easy mountiain to visit as a day trip.
Sendai, Kawasaki (world's shortest escalator, Kawasaki Daishi), Tennozu Isle, Todoroki Valley are places besides Yokohama to be explored.
Breadworks, Garden House Crafts, Aoyama Flower Market (cafe) are places you can consider visiting.