You didn't mention if this is your first time to Oahu or to Ko Olina or what time of year you will be going. That said...
Million things to do on Oahu...just most of them are on the other side of the island. As mjm1 mentioned the Diamond Head hike is completely worth it. Go the first day (while you are still on mainland time) and get there as early as possible. It is a gradual uphill hike with seating every now and then - my parents and mother-in-law were in their 70s when they made the hike the last time they came with us to Oahu. Grab a 'to go' breakfast at the Diamond Head Market (the scones in particular are delicious) and then visit Honolulu for a dose of Hawaiiana - Iolani Palace, Queen Emma, Bishop Museum, Spalding House, etc. Get a great lunch in there somewhere.
Another day visit the North Shore and in summer enjoy the beaches...or in winter watch the surfers. You can take the quick trip to Haleiwa and enjoy the beaches, shops, and eats or take the 'circle tour' - Haleiwa, Waimea Valley, Surfing Beaches (in season), Turtle Bay (beaches and golf), Kahuku (Shrimp, shrimp, more shrimp...and golf), Laie (and the Polynesian Cultural Center), Kualoa Park (and Chinaman's Hat), and then back on H3 or Hwy 61 via the Pali Lookout.
Lastly, the restaurants in Ko Olina aren't that great - particularly from a value standpoint. We did have a wonderful light dinner accompanied by music and torchlight at The Olelo Room at Aulani. That said, Monkeypod is probably the best of the 'nice' restaurants and Longhi's (onsite) was not bad either. Avoid Longboards...unless you stick to burgers and beer. But your best choice is to go into Kapolei which has many great restaurants...we're particularly fond of the Plantation Tavern.