Not sure if the FiL can do this, but the Big Island is one of I think only 2 places in the world where you can snorkel/dive with manta rays. There was an 80+ year old brother/sister who did it with us. Sister snorkeled, and brother did the dive. The snorkel is pretty easy because there is a makeshift platform you hold onto. All you have to do is kind of float/gently kick your legs.
Sunset and stargazing at Mauna Kea.
https://maunakea.com/We took a boat from Hilo down the coast to see the eruption. I wanted to see lava going into the ocean, but we missed it by about a day. Views of the lava fountains were great, but it was rough and people all around us were getting sick. And a couple weeks after our trip, a boat was hit by a lava bomb and several people were injured.
The tsunami museum in Hilo is very good (and scary), and the Japanese garden is very pretty. Tour of the candy/chocolate factory (can't remember the name) is also interesting.
There are some guided tours of the lava tubes in the park, if it's safe (ie, no eruption). We were booked for that but it was cancelled.