I want to do the Sado Long Ride, for sure. I did a loop of the north half with a friend years ago, and it was probably the best cycling route I have ever done.
I have made it through the first week on my diet and I am.... almost 4 kg down from my peak holiday weight.
The math doesn't make any sense, but I'll take it.
The north is the most spectacular. But the whole island is worth it. If you take the first ferry out and last ferry back to Niigata city. You can do it in a day.
Better still, spend 2 or 3 days and explore the mountain skyline passes.
Although hot, combining it with camping on the beach at Ogi for the Earth Celebration in August is a dream getaway.