Angelo A.
Yelp
Sapporo's Nijo Fish market is smaller than the Tsujiki Fish Market in Tokyo, but it's also less crowded, making it possible to take your time and enjoy the market at your own pace (tasting samples, chatting with the vendors, taking pictures, etc.). This is also an advantage when you're traveling with young children as we were.
Of course, there's a multitude of fresh and dried seafood: crabs, scallops, uni (sea urchin), etc. There's also numerous other food gifts, snacks and souvenirs available.
The "thing" to do, apparently, is to have breakfast there: a bowl of rice topped with salmon roe and raw uni (sea urchin). Uh, we passed on that...
Instead, on a whim, we decided to try some grilled sea urchin being offered by one of the vendors (you could also have them raw). Fun experience, as you watch the fresh uni being grilled for you. Once done, you can sit and enjoy your uni on a small bench and table in front of the stall.
Vendors will call out to you, offering you a "discount" on their products as you pass, but they're not rude or pushy as market vendors can be in other cities.
Lots to see, smell and eat. A must visit.