I agree with just about everything Xema says. I’ve been twice, both times in early January. (Last time was this year.) A couple extra comments:
Big Island Revealed Gotta have it.
Vent Trail Did it last month. As it had been raining solidly for a week, it was a challenge, but a wonderful experience. If it’s wet, just accept the fact that your feet will get soaked, even with waterproof hiking boots. I was more cautious than Xema, though, when I got to the top. People die fairly regularly on the lava fields, although I’m not aware of any deaths at exactly that point. Also, the books aren’t kidding when they say to start no later than 10. You’ll find that difficult, as you’re based on the other side of the island. You might want to consider spending one night near there, maybe at Volcano House. That’ll allow you to check out the park one day and do the vent hike on the other. Bring plenty of food and water.
Cook Monument Best way to get there is by kayak. You’ve got a good chance of seeing spinner dolphins, and the snorkelling is fine. Try to do this on a calm day, though. Launching a canoe is difficult when the surf is high at the launching place. You can mess yourself up on the rocks.
Costco If you’re not a Costco member, become one before you go. Prices are sky high in Hawai’i, but Costco remains reasonable. I made Costco stops more than once on both my trips to the Big Island, even though I didn’t stay in a condo, and therefore ate most of my meals out. (This advice applies to Oahu travel as well. Not sure if there are Costcos on the other islands.) There’s one near both Big Island airports. Some suggestions for purchases: Cooler. Hiking gear. Snorkelling gear. Sandwich makings and other food. Mixers for rum drinks and other beverages. (You’ll need to buy the rum elsewhere.) Ice. Paper plates/cups. Bamboo beach mats. Coffee, both to drink there and to bring back. Kona is way cheaper here than any other place. (In case you don’t know, there are two things grown on the Big Island that are the best of their kind in the world. One is Kona coffee.) Film. Flashlights. Batteries. (You’ll need these if you do any hiking near the lava fields, either at dusk, of it you check out any of the lava tubes.)
(The cooler’ll be $10. If any of you drink, you’ll save 10 times that price. You can either toss it, or use it as an extra piece of checked luggage on the way back. Fill it with your hiking/water gear, buy some matching color duct tape, seal it up, check it. Keep the tape handy in case it’s searched at the airport.)
Kona Airport The most screwed-up layout/boarding system I’ve ever seen. If you check luggage, expect a minimum of 4 lines. Allow 2-2.5 hours. (This isn’t necessary at other airports, at least not for inter-island travel. An hour is normally plenty. I’ve flown out of Kona twice, and both times it was a nightmare.)
Hapuna Beach Do it. Spend the day with your cooler, your beach mats.
Waikaloa Village The town, not the resort. Grocery stores nearest your
condo.
Waimaia One great restaurant, but I can’t remember the name. Probably one of the nicer ones in town and it is known for using local ingredients.
New Year’s Eve If you’re still there, expect fireworks. (although I was actually on Oahu on the 3 occasions that I was in the state on 12/31, so my experience might be skewed.)
It’s a great place. Hope you enjoy the trip.