Not all manta rays are 16 feet across, and you may not see many when you go, but when else are you going to see these amazing creatures in the wild? If that appeals to you, then this is worth doing. Recommend diving it if you are certified, it's less hectic, and you can check out the reef. Bring a rain jacket for the ride back, it gets chilly at night. With luck it will be clear and you will have an amazing view of the stars on the way back. The link here is to a site pointing out problems with safety and sustainability in their blog, which I appreciate. Also this is the dude that rescued the dolphin that was wrapped up with some fishing line. That might be a good resource. If I find a good crew doing this that is worth recommending I'll mention them here.

This would be completely amazing, except that there are so many bozos with cameras, cell phones, car headlights etc that your eyes won't have a chance to adjust to see the stars. The presentation talk does not save it. The drive up to the VIS made it worth the time for me, but if you want a great view of the stars you are better off anywhere on island where there is not a log of light. The conditions and road closure stuff on the site is for the summit road. The VIS pretty much never closes AFAICT.

You have to go. Even if the lava is not flowing that day because it is busy building the next island. My recommendation is to try find a place to stay in Volcano. That way you can make the trek from Hilo to the park, check it out a bit, stay overnight without having to drive a long way, then have a full day the next day. Obviously you are going to drive the crater rim. Perhaps less obviously, you will need to plan at least one reasonably serious hike (if you are able). The hike is necessary because like most national parks, the scale is simply too large to be absorbed. So pick a spot and spend a few hours getting into it. Bring water. Apropos getting into it, the lava tube is totally worth it. When I last went several years ago, you could walk the part that did not have lights. That's pretty neat, but watch your step. After your eyes adjust you'll find that your cell phone makes a pretty good flashlight. It's amazing close to sunset.

Described as the best snorkeling on the big island, and that's not an exaggeration. Large shallow cove (rarely deeper than 5 feet of water) with an enormous variety of fish. Nearly half the fish book present and accounted for, in fact pretty much the only thing you won't see is deep water or nocturnal. Mostly juveniles (including a few teenage turtles that like to hang out) but a few notable exceptions, especially parrotfish. Best Bird Wrasse and Elegant Coris examples I have ever seen. As usual, the water column clears up a bit if you can distance yourself from the sunscreen slathered masses near the main entry area. The less crap you leave in the water the better. There is an interesting thermocline a bit out toward the center from water running off the reef. In spots everything gets kind of wavy and then suddenly it goes crystal clear again a few feet later. The current is significant, but nothing too difficult to swim through even with backpacker fins. Be sure to mind your position relative to surge so you don't bump anything.

This is a full on beach resort (highly manicured, golf course, gated entry etc) but if you tell the entry guard that you want to go to the beach, and you are one of the 40 lucky cars or whatever, then they give you a pass and you can drive down to the public part. It is a nice beach, soft sand, easy water entry, small nicely formed waves for body surfing, rinse-off showers and restrooms. I find country club atmosphere a bit oppressive.

Two Step is the public swimming snorkeling area right next to the place of refuge national park. Enter the park and leave your vehicle there, then walk back out and snorkel until you are borderline hypothermic, then go back and check out the park exhibits while you warm up. The park has restrooms, but there are no showers or anything anywhere so you are going to be salty. It is worth it. The park exhibits are great, and the snorkeling is superb, especially if you like to dive down a bit. Huge coral heads, numerous fish, and extremely good visibility, especially if you get a little bit away from the crowds. If you are looking for a place to flop and get a tan, this is pretty rocky. The main attraction is a natural couple of steps in the rock that make getting in and out almost as easy as climbing onto a dive boat. A few keiki pools for younger kids make this a great spot for all ages of folks comfortable in the water. I've been here twice in 6 years, and it has not degraded much, but the amount of sunscreen in the water column is not helping anything. If you can protect yourself with a shirt and other alternatives, the fish and coral will thank you.

Nice black sand beach just past the north tip of the big island. This is a popular spot so parking may occasionally be tight. It's a bit of a hike down and there are no facilities. For me, this was at least as much about the scenery on the way there. Basically this destination is a great excuse to go up the north coast. The coast along the way is spectacular and the transition from dry lava fields to lush vegetation as you cross over the leeward side is equally remarkable.