The Tokyo fish market in Tsukiji is a great attraction to visit while you are in Tokyo. Many people are surprised when they ask me for my tips of what to do in Tokyo and I mention the fish market. There is the option of going in there very early in the morning to watch the tuna auctions however wandering through at a more respectable time of the morning will also reward you with the vibe and buzz of the biggest fish market in the world. You will see ocean life that you never knew existed and you will be right in amongst the action. Please take care as there is a lot of hustle and bustle, as you could imagine, as the workers go about their job to get the seafood packed and out to the restaurants and smaller markets of Tokyo as quickly as possible. For the freshest and best sushi you will ever eat there are a few small restaurants alongside the market that open up for the morning trade.
Here is a PDF file to download with instructions of how to book in for the early morning auctions. tsukijiguide The basic rule though is first in first served.