Google is your friend.
Don’t know about any stand alone gyms but I’ll bet some of the posh resorts have them. Using them . . . you will have to be creative.
Restaurants come and go. What was there when I was there may very well be gone. There are some ‘standards’ but they will be
bust as to be expected. As you intimated buying food and preparing it yourself is an option if you have a place to do so.
On a side note, take the road to Hana and do the walk to the waterfall. Magic, won’t regret it. And sunrise at the top of the volcano (dress warm).
Hope this helps. And let us know how your trip went.