Oahu only has dormant volcanos as Mikala43 mentioned..Koko Crater is another beside Diamond Head..using Waikiki as a base and renting a car,( you can us 'The Bus' for some of these venues) you can day trip to The Bishop Museum, Pearl Harbor Memorial, ( yup, as you know ANZAC troops played a major role in the WWII fighting, just don't get the merit they deserve
)
Other close by to Waikiki, but somewhat touristy things to see are Hanuama Bay, Shark's Cove on the North Shore, Waves will be kickin' in December.. (get some 'Shave Ice' at Matsumotos... (Don't know if the Shrimp farms are still selling roadside, on the way to the North Shore but the fresh shrimps are always a stop for me..)
Oh, The Byodo-In Temple..calm, serene peaceful.
If you like beaches, Waimanalo, Kailua ..beautiful.
For light snorkeling makapu'u point is nice. water is usually layed down.
Oh, Sacred Falls, and the Polynesian Cultural Center..
Jeez, so much to see, do.
Check out a Heiau, , plenty on Oahu..even one right in Hawaii Kai.. amid the houses..Agricultural one that I used to sit at first light...Heiaus are sacred places ..
The one overlooking Waimea Bay is very spiritual..Human sacrifices were performed there..follow the real 'locals' warnings regarding entry..don't wanna break 'Kapu'..
Hawaiian Royalty entered from one end..the plebes another.. I always entered from the peasants side...
I'm sure some of the 'expert' .. ahem, 'Local' wags who have lived on the Islands for a few short years will find fault in my advice..but they are what they are...
but, they 'no botha me. no botha you, Eh?'
Waikiki can be fun..knock yourself out in the ABC stores..stock up on those T -shirts/ trinkets to take back to the folks back in 'Straya.
It supports the local economy.
Hope you guys have a great trip. Hope you enjoy.
'plenny Alohas'
Disclaimer: I don't live in Hawaii, but have family living there since 1957...I have visited every year since 1985 for about two months out of the year..
If you guys need any other advice/ info feel free to DM me.
I can direct you guys to some pretty cool places on Kauai'i too if you decide to go...