There are three airlines that fly from island to island in Hawaii: Hawaiian Airlines, Island Air and Mokulele Airlines.
For some background, here is what the price differences are like for a one-way flight on a random weekday in June:

Screen grab from Google Flights.
One advantage of Mokulele is that they allow your lap dog or cat to fly with you for free, and its fleet of 11 Cessnas only carry 9 passengers each.
A bit bigger than Mokulele planes, here is what Island Air planes look like:

On the tarmac in Honolulu.
I chose the cheapest airline, a $69 Hawaiian Airlines flight in economy on a B717 from Maui to Oahu.
Maui’s main airport gives you a lush welcome, though it obviously has some heavy construction going on now:
Checking in was a breeze.
Two flights depart per hour on this Hawaiian route, so boarding and disembarking is quick.

Seats are 2-3 in coach. Don’t get stuck in seat D!
On this route Hawaiian uses what appear to be the same pre-reclined, minimal seats that Frontier Airlines uses.
I like these seats as a roomier alternative to the standard bulky airline seats:
Here is business class:
This was the first time I’d ever seen all three major U.S. airlines lined up next to each other:
Here is a time-lapse of the takeoff:
This is a ~30-minute flight, so the flight attendants started handing out juice right after takeoff.
Though I’d prefer no added sugar, at least they use Maui Natural Cane Sugar in their juice cups:
We were descending into Honolulu in no time:
The flight was short and scenic, the Hawaiian Airlines staff was friendly, and the juice was refreshing.
Can’t ask for much else.
Have you island hopped?