It first depends where you start from (where you will be landing and departing from). 6 days is pretty short if you intend to drive a lot, as you have to remember that the maximum speed is 60 km/h, so long distance journeys are time consuming.
If you are around Akan, then Mashu-ko is for me a must. I love the Shiretoko peninsula, but you won't be able to add it to your plan if you do not sacrifice something. It mostly depends on your interests : so you intend to do some hiking, visit onsen ?