The Volcanoes National Park and the summit of Mauna Kea are two different areas, so I'm assuming you mean two distinct visits. The Park is a "must see" and can easily take a full day if you're so inclined. It is a 30 minute drive from Hilo airport. There is an admission fee, per car, not per person, into the park. Once in the park, you will be able to drive around the Halemaumau pit, visit the Thurston lava tubes, see the old lava flows and calderas and then drive down to the shore where the lava is hitting the Pacific Ocean and, contrary to Will Roger's saying . . we're still making real estate in Hawaii.

Mauna Kea is the home of several world class observatorys, including the Keck. There's a visitor center at 9,000 ft for "aclimation" to the thinner air. There are also star gazing tours, but you don't get to use the big scopes, just some Celestrons that the tour company brings up. If the weather cooperated during the winter, this is one of the few spots that you can snowboad in the morning and scuba in the afternoon.