One Saturday morning, I woke up and went into Waikiki with a bunch of friends and we all rented mopeds for a day full of adventure and fun.
We were able to get around very quickly and see a lot of the south side of the island on mopeds. The cost is anywhere from $30-$40 per person on one moped or scooter. Most moped rental places open around 9am and require that you return the moped by 5pm.
I personally rented from "Hawaiian Style Rentals". They were very nice and they gave us a discount because I was with eight of my friends. You'll need to have a current driver's license and a credit or debit card for payment. They will go over the safety instructions with each person, assess damage already done to the moped and log it, so when you return they'll know exactly how much damage was previously there. Usually, they'll give you a full tank of gas, which should last all day, and each rider is required to fill it back to the level it was when they received it. Also, each moped will come with its own lock.
On our adventure, there were, in total, 27 of us riding around town, so safety is very important. There were a few crashes, no one was seriously hurt, just scrapes and bruises. You should always ride close to other riders so, if any problems arise, someone will be there to help.
Overall, this is one of the funnest things that you can do on the island. I would recommend it to anyone. I loved it!
These are some links to websites where you can make reservations to rent a moped: