Also recommend Hawaii National Park, not far from Hilo. We were there last year, the lava wasn't flowing much, maybe you'll luck out and be able to see something at night. One day is enough at the park, unless you want to camp out.
Just a few miles outside of Hilo, there is a nice surf spot where the river from the mountains, flow into the ocean(Honoli'i Beach Park) Be aware of how far out you go, the current can rip you out fast and unless your a very good swimmer, I do not recommend it.
Mauna Loa- I really wanted to do the hike from 6k feet to summit but, girlfriend wasn't too interested in starting the hike at 3 am and spending an entire days vacation walking up hill. Keep in mind, most car rental companies on the island will void your insurance and won't rent to you if you tell them your planning to drive up to Mauna Loa.(Saddle Rd. is a very fun drive, very curvy.)
Kealakekua Bay, south of Kona, is a great place to go snorkeling. Depending on how much you hike, you can either do a boat charter package where they supply the snorkeling gear and get you to the bay or you can hike down and pick your own spot. We did the hike down because I couldn't see spending money to go snorkeling when we both like hiking.
The island is very spread-out so plan your trip accordingly so you don't spend half of it zig zagging across the island.
Golf Courses- There are a few nice resorts in Kona, depending on how many days you plan on spending on the island and your budget, it might be worth bring your clubs.
I didn't, since we only had 6 days on the big island and I knew I wanted to do some hikes, go snorkeling, surf, volcano park and visit our friends who live in Hilo.
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