One of the sights on O'ahu is the Halona Blowhole Lookout. This is one of the bigger lookouts on the island and is a stop on some tour buses. During the winter whale migrations, you can see humpback whales in the distance. The Halona Blowhole is located on the southeastern end of O'ahu, about 20 to 30 minutes outside of Waikiki past Diamond Head and Hanauma Bay right along the Kalanianaole Highway (Route 72).
Speaking of Sandy's, it's off to the left (north). At that beach, there's bathrooms, showers, parking, and lifeguards. You can swim, boogie board, and surf but use caution. The shorebreak is steep, there's rocks, and currents can get strong.
It's a nice beach, just be aware of the conditions and check with the lifeguards before you jump in the water.