My wife and I have been to Oahu and Maui. You can't go wrong with either. Oahu is seems more "touristy", as least some parts of it. If you send them to Oahu, they have to go eat at
Jameson's By the Sea (great place to have dinner and watch the waves and sunset on the North shore). If they go to Maui, they have to eat at
Mama's Fish House (best food my wife and I have ever had in the US). We didn't get a chance to do it, but everyone said to go watch the sunrise on Maui at
Haleakala.
BTW, very nice of you and your siblings to do this for your parents.