When someone asks, "where do I park in downtown Honolulu," my answer usually is, "Don't." Many of the city's bus routes pass through downtown, eliminating the need for many to drive (at least in theory).
If you must drive, keep in mind that the hourly rate can easily top $3 an hour at the CHEAP garages.
Kukui Plaza was my favorite place to park when I absolutely had to drive and park for most of the day, as they offered a great $5 flat-rate early bird special for the first sixty cars. If you search hard enough, they also have some of the widest stalls I've seen in a municipal parking garage.
Key word is WAS my favorite. A few months ago, the early bird rate was eliminated. The parking management company claimed it was "too popular" and caused their workers grief from those who just missed the sixty car cutoff.
Nice of them to blame their customers as the reason for the move. Now, an 8-hour stay in the garage will cost you around $30, a six-fold increase. Monthly parking is more reasonable at $100 a month, but there is a several-month-long waiting list.