From HNL-ITO I suggest sitting on the right side of the plane. Usually the route will be such (assuming no clouds) that when you'll first see Mauna Loa, Mauna Kea, Kohala, and Hualali, and then as you descent into Hilo Mauna Kea will loom large out the window (and right now it is brilliantly snow-capped) and you will also see all the amazing waterfalls along the Hamakua Coast. If you sit on the left side of the plane you will likely see Haleakala on Maui.

(Edit: Here's a link to my flight yesterday HNL-ITO; it may give you a sense of this http://flightaware.com/live/flight/HAL262 )

Returning KOA-HNL you'll likely take off to the south, then make a right turn to the northwest. Sit on the right side for a chance to see in the distance Mauna Kea, Hualali, and Kohala (and Haleakala on Maui)

The very best volcano viewing right now is Hilo-HNL, particularly if the pilot is able to fly close to Mauna Kea. Sit on the left side.

But as slippahs points out, none of these routes will have you see the lava or steam from the Pu'u O'o vent of Kilauea.