As it is Koyou (leaves changing colour) season a very nice walk would be through the Hokkaido University campus. It's located very close to the main Sapporo Station.

http://www.best-of-sapporo-japan.com...festivals.html

Also, Otaru nearby has a nice canal that you can walk along and one of the best Japanese beers in Japan (in my opinion), Otaru Beer.

http://www.best-of-sapporo-japan.com...ing-party.html

Have a good time!