Located at 9,300 feet elevation, the Visitor Information building of the Onizuka Center for International Astronomy gives a nice place to stretch your legs, use the restroom, and become acclimated to the decreased oxygen level at altitude. People say that spending an hour here on the way up greatly reduces the chances of altitude sickness on up at the top (elevation 13,600 feet). There is also a little gift shop and, as the name implies, some interesting and informative displays are here.
Take a look at the location on a a zoom-able map by pressing "Map This" above.
And, for more information, look at http://www.ifa.hawaii.edu/info/vis/