If you plan to watch the sunrise at Haleakala, bundle up in layers as if you were wintering in the antarctic, it was 30 degrees at the summit in the summer when it was 90 degrees at sea level. also, you’ll be in the clouds where all the moisture is, so waterproof or moisture resistant outerwear as opposed to cloth coats.
There are some nice opportunities to hike, too. You can choose your level, from easy to challenging. I chose a moderate hike to a waterfall and had the opportunity to swim in a fresh water pool to cool off on the way back.
There’s also an aquarium that’s supposed to be good, but I never made it there, so I can’t give you any info about that. I thought the luaus were overpriced and overrated, but that’s my opinion.