For a similar drive/scenery to the Hana road, but much shorter try the road around the West Maui mountains ... the road is not as nice as the Hana Hwy, but it only takes 2 hrs and you see much the same scenery, though not near as much rainforest or waterfalls.
Haleakala is definitely at the top of my list, both Sunrise and Sunset are beautiful. If you go up for sunset be sure to turn your back to the sun and watch the shadows the setting sun casts of the mountain in the clouds - then hang around for it to get dark as this is one of THE best star gazing places in the world (there is a reason for all those observatories being up there - lol). If you go up for sunrise, do it your first morning while Jet lag is still working in your favor and go up early enough that you can see the stars, then watch the sky change from black to orange and red etc.. Plus doing it your first morning it doesn't get in the way of the "no fly/altitude" rules following diving.
If you enjoy history there is a very enjoyable walking tour of Lahaina (maps are free and available all over town) and don't miss out the printing house at Lahainaluna high school (part of the walking tour most folks miss) where you can view old maps, primers, Bibles and newspapers that were printed here. Remember that this was home of the first newpaper west of the rocky mountains... so it makes a nice place to stop for history buffs.
The drive thru the lava fields of South Maui is also a nice drive (heading toward LaPerouse bay) and there is some nice diving out that way as well, so you can combine a couple things at once.
Aloha, Tim
P.S. Hana is a great trip too, I plan on 10 hrs to do it justice, take your time and enjoy the rain ... that's why there are waterfalls and rainbows out there
... oh and that's also why it's a "rain" forest