Daiba Park, or Daiba Koen (台場公園), is a public park located on the sea coast of Odaiba district in Minato-ku. It used to be one of the fortresses constructed in the sea in the late Edo period (1854).
When the fleet of the US Navy lead by Matthew Perry arrived at Uraga, now part of Yokosuka, in June 1853 to force open the country, the Tokugawa Shogunate, which was the ruling body of Japan at that time, told him to come again the next year for the answer in order to buy time. So Perry went away to Ryukyu Kingdom, present Okinawa for a while. The next month shogunate decided to build eleven fortresses in the sea to defend the town of Edo, and thus their construction began at once. When Perry revisited Tokyo Bay in the following January, some of them were nearly completed. Perry avoided going further and entered present Yokohama.
Of the planned eleven forts, three were completed in April and two in December of the year 1854, but two were eventually abandoned while being constructed and the rest were never made. Now, only two of them remain, and one of them, known as Shinagawa Daiba No.3 (品川第三台場), was opened to the public as Daiba Park after it was connected to the Daiba district. Daiba district itself is an artificial land created in the sea after WWII. The other one, known as Shinagawa Daiba No.6, is still left in the sea isolated. Other fortresses were either removed or buried.
It's a 13-minute walk from Yurikamome Line Odaiba-kaihin-koen Station. You can enter Daiba Park freely without admission. You can also see both of the remaining fortresses from the Rainbow Bridge crossing the sea.
Shinagawa Daiba No.6
Shinagawa Daiba No.6 (品川第六台場), which is isolated in the sea, was also constructed in 1854. It is forbidden to land this fortress, but you can see it from the Rainbow Bride as well.
As aforementioned, you can see these fortresses from the Rainbow Bridge crossing the sea. This bridge is basically part of a toll road for automobiles (and two railway lines), but there's a pedestrian walkway attached to the bridge and you can cross the sea by foot for free (bicycles are not allowed). The length of the bridge is 798m (0.5 miles).
You can walk across the bridge from 9AM to 9PM for April to October and from 10AM to 6PM for November to March. It is closed on the third Monday of each month (the next day instead if it's a national holiday), the end of December, and the day when fireworks are at display above Tokyo Bay in summer.