Tsukiji Fish Market
While Tsukiji Fish Market is listed as Tsukiji Market in many guidebooks because, well, it does offer more than fish (and other seafood items), the thing that draws tourists to the market is the fish, particularly the tuna.
If you're willing to get up for it, the early morning magura auctions, which start around 5 AM, are an experience to witness.
If the early wake up call is too early, sushi breakfasts can still be had well into the AM hours. Before you settle down for a meal, though, check out all the market has to offer. Tsukiji Fish Market is the largest wholesale seafood market in the world, and it's offerings are varied in the most spectacular way.