There are many things to do depending on your interests.
If you like historical sites, there are many temples and shrines such as Sensoji Temple, Meiji Shrine, Kanda Myojin Shrine, etc. Imperial Palace is ex-Edo castle. You can still see the wall.
If you like nature, there are great Japanese gardens, such as Rikugien Garden and Koishikawa Korakuen. Shinjuku Gyoen National Park and Yoyogi Park, Hibiya Park are one of the biggest parks in Tokyo.
If you like amusement parks, Tokyo Disney Land / Tokyo Disney Sea is the most popular amusement parks.
Tokyo has lots of restaurants and night pubs/bars. It is very fun to explore at night, too.
If you get bored of Tokyo, get on a train or bus and in one or two hours, you can be at surrounded prefectures. There are more natures and they have local food of their own in Chiba, Saitama, Kanagawa, Tochigi, Ibaraki, Yamanashi, etc.