Go see Keck! Most powerful optical observatory ever ... until Hubble was launched anyway. I was there in '04, and if you like stuff like that you should go. It's a steep climb, but not Wolf Creek Pass or anything. Well worth it. http://www.ifa.hawaii.edu/info/vis/v...n-station.html
The Kona side of the island was cleaner and newer than the Hilo side, which seemed kinda run down to me.
You'll be tempted to buy the coffee, but it's crazy overpriced, WAY overpriced. It's cheaper at home.
The helicopter sightseeing rides are expensive too (I think $140 per person), but damn well worth it, definitely go.