In my experience the yamanote line was already crowded by 7am. The sample size is only one day in early September this year.
Originally Posted by
kevincrumbs
I'm also open to other transportation suggestions but this seems like the easiest/lowest cost.
If your plan for the day doesn't take you near Park Hyatt, maybe you can put the bag in a coin locker and pick it up in the evening on the way to the hotel.