Leather Gloves, long (denim) pants and durable long sleeve shirts, as well as a walking stick, for hiking on the lava flows on the Big Island. I still carry the rock Madam Pele gave to me... Embedded in my left middle finger. Do not go out there and get stoned. The moron who was tripping on peyote out there was shredded by her. Pele doesn't suffer fools lightly.
Best time to go... Late in the afternoon, right around the full moon... The light of the moon almost makes it like day out there. You'll still want (and need) flashlights (carry a spare). Water, cell phone, cameras (with video capability), bandages, sturdy shoes (NOT flip flops like peyote fool), and some kind of offering (watching a quarter melt is kinda neat). If you pack it in, please pack it out with you (except the quarter).
The black pincushions (sea urchins) found all around the areas you'll want to snorkel is called vana (sounds like: "Vah-nuh"). They are particularly nasty creatures. Likely imported from Japan where it is considered a delicacy, this invasive species has a really painful sting. It looks blunt like a rounded pencil led, but it has a nasty surprise hidden inside those spines. Treatment for the sting is vinegar, or urine... I was fortunate enough to be closer to the restaurant than the rest room... Of course this was the day after I had my encounter with Madam Pele, and it was (at that time) my better hand. Hurt like hell with those little spines being pressed further inside my flesh as I was having to lift and carry my luggage at the airport. I don't know what vana tastes like, but I'll make sure that other animals will get the chance every time I encounter it in the future.
Sunscreen... It's painfully easy to get burned in Hawaii on your first full day there. 15 minutes, and you can already start burning. A bad sunburn on your first day can ruin a lot of other days. Apply frequently as needed.