Flights booked!

Now I'm trying to find a place to stay... omg everything along Waikiki is so expensive!

Any tips? Maybe I'll try PriceLine...
Oh and regarding rental cars, since my plan is to not have a plan and just relax and go with the flow.
Are there car rental shops close by the main area on the random day I decide to hit up the North Shore or do I have to book in advance?