You can swim or paddle to the mini island or you can walk trough the water if you time the low tide well. Then you can hike to the top of the highest point of the island and see Oáhu´s mountains from there.
2. Mt. Olomana aka Three peaks
three ridges, each one getting steeper and more dangerous but super exciting hike. You get beautiful views of the east side of the island, mostly Kailua.
3. Ka`ena Point
The most northwestern point of O`ahu is a bird and seal sanctuary that you can get there from North shore or West side, but the west way is prettier. The walk is along the shoreline the whole time and you enter the state part through a gate that protects the Albatross and Seals, and you can get super close to them.