I’d spend a couple days on the Hilo side to do volcano and waterfalls & there’s some great zip lines and black sand beaches there.

I’d spend the rest on the Kohala Coast or Kona side for the best beaches and do the Manta Ray dive at night. I would also see if you could drive up Mauna Kea for stargazing.

Feel free to DM — my relatives live there & we’ve gone many times.