I'll second this. Parking down in Waikiki is expensive and/or a huge pain (depending on exactly how expensive you make it), and you're only about 0.75 miles away from National's Waikiki location. If you can get away with renting from down there for only part of your trip, then I highly recommend it.

If you're staying down in Waikiki for any of the days, then you definitely don't need a car those days. You can cab it to the nearer parts of Honolulu, round trip, for not much more than you'd be paying just to park. Honolulu can also be confusing to drive around since it's not really built on a grid and has some awkward sets of one-way streets.

The bus system is OK, but not highly efficient, so I wouldn't waste vacation time on it unless I was sure I'd be staying on high-frequency routes. For going outside Honolulu (so past Pearl Harbor in the east or Diamond Head in the west, or over the mountains), you'll definitely want a car.

The only downside is that the Waikiki location is only open 7 AM to 7 PM, and doesn't have after-hours drops - they'll tow the car if you leave it there. As such, you're kind of forced into renting in the morning and dropping the next morning, unless you really want it just for a day trip where you're sure you'll be back by 7.