How about Taiwan cherry (there are plenty of those in Okinawa), but there are also some other low chill cherries like kanzakura, okame, and youkouzakura that can be found in the South of Japan. 'Pink Cloud' is one that performs well in Southern California.
What about getting some fragrant varieties of camellia? Camellias are pretty popular in Japan. Or flowering quince.
If you're looking for a rarer one that's more specific to Okinawa there's Fukugi (Garcinia subelliptica), a relative of mangosteen. (in fact they even managed to graft mangosteen onto it, though it had an 85% failure rate) They make tall hedges traditional in Okinawa to help protect from wind, and the native bats eat the little ornamental yellow fruits.
Tree ferns can also be seen in Okinawa. You might want to go with Dicksonia antarctica, because it's cold hardier.
And don't neglect citrus either, the South of Japan does grow citrus. Maybe select a cold hardier variety like Satsuma. Kara mandarin is also good, but better if you have enough heat in your climate to ripen. For those extremely concerned about cold hardiness there's keraji or yuzu.
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Good ideas on path design, rockwork, and use of palms.
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