Has to be Awaji Island for me. I went there this summer and stayed at the convention center that's next to the garden memorial. One of the most beautiful gardens I've seen in my life.
To get to the island you have to cross the longest extansion bridge in the world from Kobe
I recorded a part of it on camera, you can see it
YouTube - Japan Travel Lounge Music Video FULL HD at 3:39
Actually in the off-season, (i went in July). The price of the hotels are super cheap, I paid 129$ per day for a room which is normally 350$ during the golden week.
Next target = Okinawa!