The hike up Diamond head was beautiful and very busy. We went first thing in the morning, prior to the park opening about 545am. Our tour guide was awesome, full of information, kind, and friendly. She got us through the line and dropped us off at the entrance and waited for us to finish and take us back. We were essentially paying for an Uber with far better conversation, information, and a perk or two.
I would go on our own in the future, but definitely early with plenty of water.