I have done the para-sailing, that is fun.

I have done all of the Luaus, and I recommend the full day trip to the Polynesian Cultural Center with a guide. This is an all day event which includes a luau, but it is the "Must See" attraction on Oahu for tourists. Don't let the crufty name fool you; this is a Hawaiian themed park, not a museum.

If you don't want to do the all-day thing, or must have alcohol with your meal, then I recommend the Paradise Cove Luau.

Snorkelling isn't that great on Oahu, but the best places I have seen are Hanauma Bay, Shark's Cove, offshore of the Kahe Power Plant and in front of the Aquarium in Waikiki. Hanauma Bay is super crowded usually, as is Shark's Cove, but these are the best. Off Waikiki is not as good, but if you are staying in Waikiki it saves a lot of effort in travelling.

I personally didn't think the Circle Island Tour was worth it at all. It is basically a shopping trip at tourist traps around the island. You can do the same thing for a $2 bus ticket.

Also: The Dolphin Submarine is really cool.
Catamaran "booze cruise" on waikiki beach is awesome, get the one with free drinks! The Byodo-In Temple in the Valley of Temples is a nice hour visit which will stick in your memory. Visit if you are go to the Kaneohe side of the island.

What not to do: I think the the Sealife park is extremely overpriced and not very good. I recommend only eating at restaurants along the beach in waikiki (not on kuhio), or the restaurant row on Ala Moana. There is a good chance of food poisoning elsewhere. Do not get trips from people you meet on the streets. If they don't have an office, their services are probably terrible. Do not let someone hand you a parrot, do not hand anyone your camera.