Frank72 already mentioned the only route suggestions I know for the Big Island (Queens Highway, going either north or south), but the Bike Works Kona shop has a list of routes on their website:
https://www.bikeworkskona.com/suggested-rides/
btw not sure if they still do this, but Bike Works Kona used to host some midweek rides that were a pantload of fun. Although, as I posted on another cycling forum
My wife and I got hooked up with a Bike Works Kona midweek group ride when we were there on vacation in 2013 (we'd brought our own bikes) and it was one of the most fun -- albeit, perplexing -- pickup rides I've ever been on.
I think most of the locals who typically turn out for their shop rides are triathletes, and so the notion of a cooperative group ride is completely foreign to their experience: Someone would get to the front of the line and just go as hard as they possibly could until they popped and got spit out the back; next person in line simply repeated the exact same process.
At the midway point when we stopped for a break I taught three other riders the concept of a cooperative rotating paceline, and after a wonderfully brisk/spirited return where we all got back to the shop together, they couldn't shut up about how cool this new concept they'd just learned was!
