I've never before experienced anything quite like the Tsukiji Fish Market - all manner of things in tanks and on trays of ice, scooters zipping around, the amazing sushi at Sushizanmai, a restaurant at the market, the working people going about their daily business - such an eye opening experience and I'd say my highlight of Tokyo. Unmissable - get there early.

Do not visit Tokyo without taking a trip to the almost daily fish market.
Arrive at 5:30am (preferably after a night out drinking!) and head to the tuna auction. Wander around the various other auctions & the general fish market. Finish up with a delicious sushi breakfast & beer at one of the surrounding restaurants and retire to bed. Definitely not to be missed.

How big is a tuna? What to shrimp look like before they get on your plate?
I'm not a seafood eater but I still have a very interesting and memorable morning at Tsukiji. You can get there at 6am or even 7am and as things are winding down. There is talk of closing off the famous tuna auction to tourists because there are getting to be too many (tourists, not tuna).
As you walk around, remember to warch out. The workers zip around on carts through spaces that look too small for access. Also, I highly recommend heading to the back in search of the "living fish". There is a warehouse with tubs of seemingly every kind of sea creature. This is also where you can find the workers with buckets full of live fish. Something you never see at the supermarket...

Before you go call or check the homepage so you know when the vacation days are that month. The first time I went no one was there!

Caught the first Subway train of the day to the market.
Despite popular opinion there is still plenty to see at this time.

Lots of weird sea creatures.

Tuna auctions was a bit difficult to find, until we decided to follow the carts of tuna in reverse.

Wow, there really was some monster tuna there.

Good photo ops with the ice mist low on the ground.

The market guys were surprisingly tolerant of a bunch of dumb tourists stumbling around while they tried to make a living.

Sushi breakfast in one of the little restaurants next to the market seemed a bit rich for my tastes so early in the morning, but was worth it as the tuna was sublimely good.

Recommended.