Mount Fuji: While not technically within Tokyo, Mount Fuji is a popular day trip destination and offers stunning views.
Mt. Fuji Climbing - Official Web (fujisan-climb.jp)
Odaiba: This artificial island offers waterfront views, parks, and the Rainbow Bridge.
Odaiba (Daiba) - Tokyo Travel (japan-guide.com)
Shinjuku Gyoen National Garden: This beautiful park offers a variety of gardens, including a traditional Japanese garden.