Otaru is definitely a must visit, especially in winter.

The view from Okurayama is good, but I actually recommend the ropeway to the top of Mount Moiwa. For that matter, the view from Tenguyama in Otaru is actually really good too.

In terms of day trips, if you're a fan of onsen some of the most famous ones in Japan are located a two hour train ride south in Noboribetsu.

Food: if you're going to do sushi then you'll want to eat in Otaru, it's cheaper, larger and fresher. Most kaiten sushi restaurants are really good quality and fairly inexpensive (starting at a little over 100 yen for two pieces). Ramen is also really famous in Sapporo. Depending on where you go either shio or miso will be the speciality. Finally, my personal favorite is Genghis Khan (ジンギスカン）essentially grilled mutton with vegetables served over or with rice.