The summit of Mauna Kea is like another world compared to what you've probably seen of Hawai'i. If you find yourself on the island, this is a great place to visit. There is a visitors' gallery at the Keck Observatory, but please note that its hours are limited to Monday - Thursday, 10 AM - 4 PM last time I checked.
-- WARNING --
Please be aware that with nothing growing up on top of Mauna Kea, the soil is very loose. I found that when I wandered around the area for only about half an hour, it took almost two hours to get back up to the top, so please be aware of this if you feel like wandering away from the beaten path.