Hi, there, I traveled Hokkaido several months ago, I based in Sapporo and made navette to Otaru, Noboribetsu, etc. If I remember, from Sapporo central train station to Otaru, it took me less than 40min.. maybe 30 min. It depends on which train you take, there is express and regular train. I did one day trip to Otaru from Sapporo, where I booked for my hotel for whole journey. But if you stay one night in Otaru, it is not bad idea. Anyway, Otaru is small town where you can see only one long street where you can find many shops, museum, craftsman's workshop studios. What I recommend is get off from minami Otaru station, which is one stop before Otaru station. From Minami Otaru, you walk toward Otaru,(it is just one main street), you can finally reach the Otaru canal, which has nice view at night. The pass which I bought was Sapporo-Otaru welcome pass which allow one day round trip. That is cheaper than to buy one way ticket, two times. You can buy at the airport, information center. But if you stay over night, no need to buy this pass. FYI, I strongly recommend to enjoy hot spring spa in Noboribetsu, if you like hot spring.