Mauna Kea observatory, with the Keck telescopes in the center, Subaru telescope at the left, and NASA infrared telescope to the right
Mauna Kea is an inactive volcano on Hawaii with heights of roughly feet. With its high altitude, dry environment, and stable airflow, Mauna Kea's summit is one of the best sites in the world.
Tours – I really like the people at Hawaii Forest and Trails. They do a number of tours including the observatories.