You can drive up to the Visitor's Center, which sits at 9,200 feet elevation. If you choose to go to the summit, you are required to have a 4WD vehicle to travel to the summit at 13,800 feet, and the drive is very difficult, and it is very cold at the summit. However, it is one of the best stargazing sites in the world if you are feeling adventurous.
Wailuku River State Park is worth some of your time when you're in the Hilo area. Wailuku River is the longest river in Hawai'i (both the state and the island) and its course lies mostly along the divide between the lava flows of Mauna Kea and those of Mauna Loa to the south.