^ You're on the right track. Honoapiilani Highway (including the partially-completed HI 3000 Lahaina Bypass) is unexciting, but it avoids backtracking on the one-lane county portion of the Kahekili Highway.
I've seen, from the Honoapiilani Highway, what might be a hikeable old alignment, in the Olowalu Tunnel area. But I've never tried to check it out.
Standard tourist advice is to do the Haleakala summit road to catch sunrise on the mountain. I did that once, waking up at 2am. My later visits have been at a more reasonable hour (sometimes around sunset, some not), and those worked too.
The Hana/Piilani loop takes you by the much overrated "Seven Sacred Pools" (more than seven, none more sacred than any other pools). I'd stop there for a pee break, and also check with a ranger about road conditions on the unpaved and sometimes impassible part of the Piilani.