On the big island the whole time?
Volcano is going off so be sure to see it.
If u drive near the army base in the middle of the island at night, you can often see military practising with tracer rounds. Really neat to watch.
Manta ray dive is epic. I would 100% recommend it. U can do it as a snorkel as well.
Waves will be the biggest on the north shore of the island. U have a decent chance of seeing some big waves roll through this time of year. Didn't surf much on the big island, but if youre going to try, head to the west end of the island (kona). There may be enough wrap around for good surf to learn on. Plus its on the lee of the island so the water tends to be smoother.
If you are on oahu, I can give u much more info. I lived there for 4 years