When Adele was younger she had TONS of energy, so I taught her to "run up stairs." I'm not sure if it burnt off much energy or not (that was the idea), but she would run up and down the stairs. But she only weighs 50lbs, so it might not work for an Akita.
Does Zero really like to fetch? That's the easiest thing I can think of unless you have a dog park nearby. Then he could run and play with other dogs, and you could just bundle up and stand there and watch (or play too).