Great tour up to the 13 Observatories (no entry allowed) at 4000m as well as the sunset over the whole of Hawaii. The tour guide was fantastic - explaining many things along the way, providing gloves and a warm jacket, snacks and photos. They also provided views of Jupiter, Saturn and the moon through an medium sized telescope.
There is a compulsory half hour rest at the Information Center to acclimatize to the altitude before ascending to the Observatories.
Bring plenty of water and/or Diamox to avoid altitude sickness.