Since Mauna Kea is a shield volcano, it's eruptions are gentle and quiet. In fact, even beautiful. Lava slowly flows down, and when the eruption is finished, the lava cools and forms a new layer of magma and crust mixed together.
An eruption would do little or no damage, as eruptions from Mauna Kea are not violent, unlike composite and cinder cone volcanoes, which erupt abruptly, sending lava, ash, dust, and rocks into the air, restricting vision and aerial transportation. That kind of eruption would do a lot of damage.
Mauna Kea is dormant and has not erupted for 6,500 years.Edit
Shield volcanos are usually extremely blunt, sloping just barely, like a shield (hence it's name). They are mostly easily to climp, needing little or no equipment. However, the large area of the volcano means that it'll take quite a long time to climb.