Cairo Carol’s recommendations are excellent.
I’ll add a few more observations from my most recent visits (2010 & August 2012).
Saddle Road is a piece of cake now. It’s been straightened and repaved. Most rental car companies don’t restrict you anymore. I know Dollar and Alamo do not. They still don’t let you go to the Mauna Kea summit as you do need 4WD to go past the visitor center. They have telescope and sky parties at the visitor almost every night. If your trip coincides with the new moon, definitely go (and dress warm!!).
Hawaii Volcanoes National Park - Make time to hike the Kilaueua Ikitrail. It’s a 4-mile loop that goes through the rain forest along the crater rim, then descends 400 feet to the crater floor and walks across the hardened lava from the 1959 eruption. Well worth the 2-3 hours spent. Doing the loop counter-clockwise makes the ascent from the crater floor much easier.
If you have more time, and will be in the park on a Wednesday, call the visitor center a week ahead of time and sign up for the Pua Po’o Wild Lava Tube tour. They don’t advertise it much and it’s limited to 12 people per week.
In Puna (Hilo Side), the Kapoho Tidepools are a fun snorkeling spot. Not as rich in sealife as on the Kona side, but the lava makes a natural breakwater that keeps the water calm. Nearby is the Champagne Pond, another tide pool that is heated volcanically. We rented a house less than a block from here last time.
The submarine tour in Kona is a little pricy but fun. My kids loved it when they were 10 & 8.
This last time we tried the Kohala Ditch Adventure. The Kohala Ditch is an irrigation canal built at the turn of the last century to bring water from the mountains down to the sugar plantations. It’s still used to irrigate some farms, but the sugar plantations are all gone now. This company takes you up the side of the mountain in a truck, puts you in the ditch in an inflatable kayak, and you spend the next 2 hours floating through the rain forest, over trestle bridges, and through hand-carved tunnels with only a headlamp providing light. You will get wet. Again, it’s pricy (even after the on-line discount and child pricing it still averaged $110/per person) but fun.
A few recommendations for Waikoloa. If you like cats (and I think it’s required for all Dopers to like cats) A-Bay (the beach in front of the Marriott) has a large, mostly friendly feral cat colony in the trees on the south end of the beach (near the public parking lot). The cats live in the trees and the lava caves. The locals leave out food. My sons liked to bring their dinner leftovers to the cats. Many come over to be petted, then stroll back into the bushes when they’re done. (On the other hand, the feral cats at the Hilton are your typical run-away-from-people scrawny kitties).
You don’t say where you’re staying, but in Waikoloa, the choices are pretty much Hilton, Marriott, or a condo rental. In any case, you’ll have a refrigerator. If you choose to stock it, drive the 10 minutes up the hill into Waikoloa Village and shop there. Much, much better prices than in the Queens Marketplace grocery store. Queens Marketplace does have a pretty good food court if you just want a quick snack.