the best way to get around in Hokkaido is Public transportation, including train, bus and taxi.
don't recommend it for the entire stay, but for some parts of the trip, where public transportation is inconvenient, it might be a good option.
Inside Sapporo, however, it is probably better to use taxis instead of buses and subways, because walking and some stairs is often involved when using the subways and buses.
Otaru is relatively easily accessed by train, but for medium and longer distances within Otaru, a taxi might again be the best way of getting around.