Here are links to two things on the big island-
1. See lava at Kilauea,
http://hvo.wr.usgs.gov/kilauea - If you go at dusk (bring binoculars, flashlight and water), you will see the orange lava. Going during the day you will see steam (when lava hits the water). Walking on the old lava flows is precarious and dangerous (sharp rocks) - wear gym shoes. Safe paths will be identifed by the US Forest Service. Volcano info center - 985-6010
2. Visit Mauna Kea observatory (ONIZUKA CENTER FOR INTERNATIONAL ASTRONOMY) -
http://www.ifa.hawaii.edu/info/vis/ each evening, volunteers set up telescopes for visitors to see stars/planets. Visitor information center: Altitude: 9,300 ft.
International observatories: Altitude: 13,000 ft (bring winter clothes, it gets snow in January). The observatories here are not open to public, only scientists use these telescopes. If you drive to top, you will need a 4wheel drive vehicle. Check with your rental car company about driving on Saddle Road to visit the Observatories. If you can't there are tour vans that can take you to the top.