Maui - You will need to make advance reservations for the "Old Lahina" luau well before they get there. It is the only one I would suggest they go to and it is VERY popular.
Aquarium.
Bike Ride down from the top of the volcano is a coast all the way down and takes half a day. You have to get up VERY early for the bus ride up to the top but the sunrise makes it worth the effort.
You can drive up the coast to Hana and stop along the way to see the jungle sights.
We stayed in kehei and had a view of the volcano from the bed in our room. NICE!
Catamaran trip from Maui to Lanai is a nice excursion.
Hope they enjoy the trip!
-Mark.