If you're looking for a great izakaya (Japanese style bar) or a big fan of seafood, you should try Isomaru Suisan. It is a causal chain restaurant often found near stations in Tokyo and the cities. The fresh seafood grilled in front of you is amazing! It's a place where you can order seafood and grill it yourself in the tiny grill they provide: clams, scallops and squid.
Sapporo Wholesale Market ("Sapporo Jogai Ichiba" in Japanese) is one of the major points of interest in the city. It's one of the best spots to try fresh seafood Hokkaido is famous for (the place is also called Sapporo Crab Market). Here's a guide to the market that includes the best places to eat Hokkaido's specialties!