Volcano National Park was the highlight for me on the Big Island. Allocate sufficient time for this. You can't see this anywhere else.
Avoid the Hilo-side of the island because it is too wet and grey. Also avoid the falls on the north side of the island. The locals rave about the falls, but they are pathetic compared to what we have in BC.
Don't cheap out on the car rental. Rent a convertible car or even a Jeep -- it is worth it.
If you like coffee, you may become addicted to Kona coffee. I brought some home with me.
The observatory can only be reached using a 4x4. I found the observatory, itself, to be quite boring.
At night, you can see the lights on Maui from the north tip of Hawaii.