If it is just for resting, I think the main island of Okinawa is probably the best affordable deal... There are plenty of resort hotels on that island.
On the other hand, if you do want a more exotic destination and don't mind paying a little more to access the place, Iriomote island - one of the southern in Okinawa - is really a fantastic uncrowded destination. Check the guide on this web site :
http://www.secret-japan.com/forum/viewtopic.php?t=55
sanji
[MOD: Topic moved to discussion]