tapioca

Go to Pearl Harbour to see the sink ships.
Try out foods @ shrimp truck on the road, very delicious.
Swap meet is cool but only two or three times a week when there is no event happened at the Stadium, so check schedule before you go.
Hike the Diamond Head and you can see view of Waikiki from there.  
Didn't see anyone doing segway over there, I would rent a care to drive around the whole Island. Like other lyk said, there are lots of beautiful beaches in Ohau and have less people.