There isn't much to see in the city, it would take a day and a half. But I would do it, the seafood there is incredible and getting a Genghis Khan dinner at the Sapporo brewery while drinking beer they only serve there is fun.
https://www.sushi-hanamaru.com/la_en/ The Numero in the Sapporo train station is incredible, if I'm being honest on par with a 1* omakase restaurant I went to in Kyoto on the same trip last October. Went back in May with my dad and he loved it as well.
Taking the train to Otaru is easy, ~1hr on one train line, and would be a good day trip. If you're into sake the Tanaka brewery has a short self-guided tour and then tasting.