Okay, I have two don't miss activities for you. Whatever you do, don't miss either of these. Some of the big island's most spectacular activites are NOT in kona, so don't spend all your time there (in fact, I haven't been to kona yet!):
1. Stay at least one night at Volcano House, better 2 or 3. Travel agents don't book Volcano house, so you'll need to do it yourself. Do not miss it -- it is on the edge of an enormous caldera, the view is unbelieveable and unmatched, and you spend the next day hiking and driving around the caldera seeing amazing things. Pay the extra $ to stay in a room on the caldera side, the side with the view.
Some quick trips from there: find out where the eruption is. If it's close, check it out. If not, just drive to the end of the road where the ocean is and check it out. Also, off that road, about a mile hike, is a pretty cool collection of petroglyphs carved by ancient Hawaiians in the rock. A good travel book will have details on how to get to both, though I can dig up info if you can't find it.
2. The other must, especially for a flashlight freak: you must find a wild lava tube and hike in it. Not the Thurston lava tube, which has been cleaned and has a walkway through it and is for tourists who impress easily, but a real wild tube [quick note about Thurston tube, though: there's a metal gate at the end of the tube, which is unlocked; if you bring flashlights, you are allowed to open it and continue down the tube]. Going through a real wild tube is one of my wife and my favorite vacation highlights of all time.
Some very good tourbooks mention where the wild tubes are, but if you can't find mention, let me know and I'll dig the info up. Here's what to expect: you're hiking in pitch black darkness (can't see your hand in front of your face if your flashlight is off), on top of sharp shifting lava rocks. The walls are adorned with all kidns of cool mineral deposits. There are a few sections where the formations get tight and you have to squeeze through, but getting through to the other side is worth it. Through the hike there's liquid leaking into the tube and you can imagine the lava pouring through it many years ago. The entire effect is fantastic and almost surreal.
Here's what to take. Take two flashlights each, because if you lose light, you're hosed. Lava tubes are made up of black porous rock that "eat" light, so expect your lights to look much dimmer than usual, even though it's pitch black in the tube. Take water, and I also brought a basic first aid kit in case someone slipped on the rock.
Many people show up at these tubes without a flashlight, so they can't hike in more than 10 yards before they have to turn back, disappointed. You'll be prepared!