If you are planning a day trip to Otaru from Sapporo then I recommend taking train. Rapid train runs between Otaru and Sapporo every 30 minutes, train takes about 40-45 from Sapporo to Otaru, and fare is JYE 620 one way. Pretty much all tourist sites are walking distance from Otaru station, I done that.
What I liked about taking train to Otaru is a train runs right by the ocean with a nice view and do not have to worry about having beer and sake while touring Otaru.