Personally, I would not split up your trip to Oahu.... you will waste time returning a car... checking out... and flying between islands.
I would do Oahu and then Maui if those are your island choices. As always, I will say you are missing true Hawaii if you do not do the Big Island!
Get your car now... I am not even sure you will get that Enterprise rate in Hawaii.
I think the budget is unrealistic... as Doc said, you will use up much just for parking.
BTW.. what $10 hotel did you get?