The common touristy stuff includes:
Arizona/Missouri memorials, Diamond Head, Sea Life Park, harbor tours/glass bottom boat, charter fishing, submarine tour, Hanauma Bay, Waikiki Beach (usually crowded, and dirty IMO), PCC Luau, etc.
Personally, I prefer the North Shore (and maybe why Island911 was asking if they will have a car?). Much less crowded, and much better scenery/beaches (Waimea Bay, the falls, Turtle Bay, etc.).
If surfing/surf-watching is your thing (December will be a great time): You can't beat the "7 Mile Miracle" of the North Shore. From west to east: Haleiwa, Lani's, Waimea, Three Tables, Log Cabins, Pipe/Back Door, Sunset, V-Land (plus a bunch more in between).
Either way, good on ya. I am sure they will love it.
