There are a heap of charter boats out from Ohau which is where the international air port is so I am sure that you could organize a trip once there plus it has the benefit or having a great outlet centre that your wife could spend time in. The outlet centre is the size of a mall and takes all day I can tell you.
If you go to Maui, again there are a lot of charters but with a lot less populace it would take more to organize. The down side is that there is less things probably for your wife to do.
When you have more plans as to how long your staying in Hawaii or which Islands we may be able to help further. My advice would be to fly to Maui or another Island ASAP then come back to Oahu to finish the trip.
You can do a submarine trip, at trip around the Island is worth while as well.
Personally, I would not look at hiring a car in Oahu but the other Islands are fine as the less population makes things easier. Your normally upgraded to a Mustang convertible or similar so just book a smaller car and see what happens