It's easy to forget that Japan is an island country if you're living in Tokyo. I've often heard it said that Tokyo harbor isn't exactly the body of water that you want to dip your toes in, anyway. However, there is one small stretch of clean, undeveloped sea shore. Kasai Rinkaikoen is a great place to go and remember that the ocean is really within arm's reach. The park is expansive, and offers enough to easily fill an entire afternoon. First, be sure to stop at the park service center, which is just inside the park. There you can find maps and brochures in English. It's helpful to have a map because the park large and there a maze of intersecting walking paths. After that, the View Point Visitor's Center, or Crystal View in Japanese, is a nice place to start your tour of the park. A large modern rectangular building, it's basically a glassed in promenade. Be sure to check out the displays explaining Kasai park's history and wildlife.
From there, walk down to the Nagisa Bridge and head over to the Kasai Marine Park. I had a blast in this part of the park. Here you can take your shoes of and walk in the surf. When I was a kid, I remember playing in tide pools at the beach. Since all sorts of little critters get caught in the tide pools, they are fascinating places. The Kasai Marine Park is almost one big tide pool. Little minnows dart around in the shallows, little crabs burrow in the sand, and little birds run around the beach trying to catch the minnows and the crabs. I had a National Geographic moment when I got to see two little crabs fighting. As always, nature is an adventure. Visitors to this beach are advised to stay in the shallows instead of swimming.
The park is full of interesting birds. My visit to Kasai park made me regret that I don't know much about Japanese birds. While you can see birds swooping over the entire park, almost a third of the park is devoted to bird watching. There are two large ponds in that part of the park; one with brackish water and one with fresh. At various points around these ponds, there are several birdwatching houses and a larger birdwatching center.
Notably, the park features a large ferris wheel. It turns constantly, but slowly enough so that you can easily hop in and out. The entire circuit takes several minutes. Be sure to take a friend, and then sit back and drink in the view. Though Tokyo tends to be obscured by haze, the view from the top of the ferris wheel is nice.
Also, be sure to visit the Tokyo Sea Life Park, which is on the grounds of Kasai park. If you've already bought a ticket to the ferris wheel, you'll get a discount if you present it at the Tokyo Sea Life Park ticket window and vice versa.