Since you are on the mainland, I would stay there for that budget as there is so much to see and do there. Okinawa has wonderful beaches that are more enjoyable in warm weather which ofcourse is my opinion. There r hikes as well but again, for your budget it would not be feasible to fly here and no transportation on island except taxi which is pricey and bus. Bus is limited in where it goes I think too. Hope this helps.