Maui, I suggest a Villas at Kapalua for all 4 days. You can do a different activity each day if you even feel like leaving your villa. They are rented though the Kapaula Resort hotel and are nice and ocean front w/private beach and not terribly expensive.
Went to Puerto Rico last year and did the whole island, and there are some nice ecosystem sites there & old San Juan/fort is interesting / Biolumenescent bay has to be one of the coolest things on the planet / james bond radio telescope / beaches in Rincon are nice, but its a far cry from Hawaii. Its really an impoverished place for the most part and driving a car there is an experience in itself.
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