The previous post is good advice. If you can make it to Maui, the sacrifice of time and expense will be well rewarded.
When I lived on Maui, I used to hike into Hosmer's Grove area and beyond every weekend. My advice to anyone else hiking on the slopes of Haleakala would be to go alone, hike deep into the forest (beyond Hosmer's), find a secluded and comfortable spot, preferably within an area composed of mostly Ohia and Koa, if you can find it, and to stay there, motionless, for as long as you can. Do not be discouraged by the lack of birds at first. Eventually, you will be treated sightings of birds that exist nowhere else on the face of the earth but Hawaii, and sometimes birds that exist nowhere in Hawaii but Maui. You will see, hopefully, Iiwi, Apapani, Amakihi, and, if you are very lucky, some Maui Creepers (but Maui Creepers probably only if you are in a Koa grove).
I always used to wonder why the evolution the Iiwi and the Apapani caused them to have bright red plumage (even females). I am no expert on the topic, but, I think that I might have found my answer when once, when I was trying to find individual examples of either species while observing from the rim of a valley, looking down at the canopy of a forrest of nearly 100% Ohia Lehua. Although many Apapani and a few Iiwi were within the forrest, I found it nearly impossible to see any of the individual birds of these species unless they were in flight.
For those who are not familiar with Ohia Lehua, the tree has drab green leaves, and has bright red blossoms. So, the top of the canopy is, essentially, drab green punctuated by many bright red blooms. Therefore, the bright red appearance of these birds is actually a perfect camoflage within their native forrest. If I were a hawk looking for lunch, or if I were a human hunting one of these birds for their beautiful plumage (as did native Hawaiians), these bright red birds are nearly impossible to distinguish from the bright red Ohia Lehua blossoms unless and until the bird takes flight.
For all I know, my theory above might be pure folly (and maybe a more knowledgable person can correct me). However, I think that it is interesting that a bright red bird that I thought would be so easy to spot turned out to be nearly impossible to find within their natural environment that includes bright red blossoms of approximately the same shade of red.
P.S. See HawaiianOnline's excellent photo of an Apapani in an Ohia Lehua tree to see an example of what I discussed above. HawaiianOnline, you have taken many excellent photos, and I enjoyed viewing them.