Hello from gorgeous Northern California -
My favorite Hawaiian island is Maui. You cannot possibly get bored - if you don't like the climate, view or activity in one area a short drive takes you somewhere completely different (beach, upcountry). I also think Maui is very manageable if you have a short time there, which to me is 5 days. Most people seem to stay in the Kihei/Wailea area, but to me it's too hot. I prefer Napili where you don't need air conditioning every day and can enjoy the tropical breezes, plus you're not in the middle of high rise hotels or condos.
Any place you choose in Hawaii will be a great vacation, but for me, Maui no ka oi (Maui is the best!).
Joli San Rafael, CA