If you do want to sightsee, it will be much easier with a car. One major benefit is you'll be able to do things at your own pace. Oahu isn't big; without stops, you can drive around the island in about two hours. That said, there are things you might want to see and do that aren't accessible by public transportation.

Yes. Renting a car at 18 years old is as easy as renting a car as any other adult – all you need is a Hui account.

You can take The Bus, the trolley, a taxi, or rent a bike to do a lot of sightseeing. Renting a car for your entire trip isn't entirely necessary, especially if you don't want to pay the expensive parking fees that many hotels charge. You can explore downtown Honolulu easily by hopping on The Bus.