Just in case anyone searches for this with a similar question, an update on my trip-
I ended up not renting a bike. I called a place called The Bike Shop in HNL and they had a road bike in my size for $45/day, and if I'd picked it up on the 24th I could have had it through the 26th because they're closed on xmas and wouldn't charge for the extra day. Seemed like a good deal.
Still, I didn't feel the need because I ended up spending 95% of my waking hours outside either hiking, walking, or swimming. I spent a few pre-dawn hours on the first couple days riding stationary bikes in the hotel gym, but that was more just because I hadn't adjusted to the time change than anything.
We hiked Diamond Head, which was crowded and not very tough, and Koko Head, which is an endless stair-stepper that doesn't seem that bad until it starts raining and people all around you are slipping and sliding down a few steps at a time. I did it quickly and can now barely walk 3 days later thanks to sore legs.
I also hate running but found myself going for long runs just because it's such a beautiful place and you keep turning down different streets without wanting to go back. Snorkeling and ocean swims are also obviously great.
I did see plenty of road riders on the highway that heads north on the eastern shore from Waikiki. Traffic and the width of the margins on the road both seemed totally acceptable for good rides, so I could see myself enjoying a bike if I ever go back. This is what Christmas sunrise looked like from Koko-
