Lessons
As I've already mentioned, we might have seen more of the west coast of Hawaii - for example the Captain Cook obelisk - had we spent a night or two over there.
Like the big island, there are obviously lots of out-of-town things to see on Oahu. But it's difficult to get out of Waikiki / Honolulu without a car.
Certainly public transport is cheap around town. A day bus pass costs $2.50 (correct change only on busses) but the traffic is slow and the stops frequent and then there are the ponderous traffic lights. Why are the cycles so long? So its interminable to get anywhere. It's made more difficult to use the busses if you're unfamiliar with the routes. There are no maps on the busses and several different providers. Using a mobile phone to check Google Maps is a solution but requires mobile data.
So it's easy to spend an extra hour, beyond the actual travel time, waiting and/or walking to bus stops. And unlike driving, riding a bus is boring and it's often uncomfortable, particularly if you have to stand.
Uber is an option if you can find an open Wi-Fi spot to call, but it's not really practical for a casual drive around to see what one can see or along the coast or to take a look at pineapples growing.
Looking back we agreed that it was a big mistake not to rent a car on both islands and to simply pay the additional cost of hotel parking in Oahu. As it was our visit to Pearl Harbour was cut short due to our dependence on a shuttle bus to come and go. And we would certainly have seen a lot more of the island with a car.