Seaside Top is the observation deck of the World Trade Center Tokyo, a towering 40 story building. The Hamamatsu-cho subway station exits directly into the building making it easy to exit, pay the ¥620 fee and hop in the elevator.
At the top you'll find a unique view of Tokyo Bay, other skyscrapers like the Tokyo Tower and Tokyo Sky Tree and even Mount Fuji on a good day. Display screens with light up buttons help you to determine exactly what you're looking at. The deck is usually not crowded so you can linger and enjoy the 360 degree views. The night view is considered particularly romantic, but keep in mind that the deck closes at 8:30.
Open daily 10am-8:30pm, doors close at 8 pm. The building itself is connected directly to Toei Subway Daimon Station and Hamamatsucho Station Keihin-Tohoku Line and Yamanote Line. There is also the Tokyo Monorail Station nearby. Fee is 620 yen per person.