I've been on tours though,... I was in the business of Hawaii tours up until late August.
The only (guided) bike tour I'd do on Oahu again, would be the 100% Mountain bike tour
(link goes to my photos) put on by Bike Hawaii, and possibly their hike/bike/snorkel tour.
Been There... Done That...
If you click here, you can see the Slide show
I've done other tours put on by other companies on Oahu, and I can't say I was very happy with the product.
On Maui, I've done the Haleakala Downhill tours twice (got to see the sunrise both times), but that was "back in the day" (before I had a digital camera). Now the ride portion begins outside of the Park, and I've yet to do the tour with the wine (not whine) stop. Given half a chance, I'd try to ride from Paia to the top (and back) which comes out to be ~76 miles, with a summit at 10023' (at the halfway mark
Never got a chance to try the Waimea Canyon tour in Kauai.
Nor any kind of bike tour on the Big Island.
Still, I want to go back, and circle each of the inhabited islands. There is a book of self guided tours (Hawaii by Bike). However, it was written before 9/11, and I'm a little concerned about how dated it was.
Anyway, Have a great time! (bring me back a tee shirt