From the Tokyo's Best Stuff
Odaiba is a newish part of Tokyo which has mostly come into existence through the Japanese-perfected process of reclaimation (in Odaiba's case, from Tokyo Bay). After some years of little direction, the area is now booming and looks likely to be a well-lit and clean pleasure destination for Tokyoites. Kind of like a huge Darling Harbour, for those who know Sydney, except looking more like the result of an acid-soaked pre-schooler's architecture class. But what the hey, it's a fun place.
To get to Odaiba take the Yurikamome monorail and make sure you get the front carriage, as the ride is pretty trippy. At one stage the train loops over the bay by what seems like levitation. You'll get off at Ariake Station. Get the monorail back into town at sunset so you can admire the stunning Rainbow Bridge over Tokyo Bay.
You can also walk over the Rainbow Bridge, although it's a bit of a slog for unfit geeky folk. There are observation decks on both sides of the bridge to pause and catch your breath.
Tokyo Big Sight is one of the aforementioned tripped-out buildings. Enjoy the annexe with the huge carpenter's saw sticking out of the building from the observation lounge.
Check out the Sega Joypolis down by the water at the Decks promenade.