Best tip for rental car. Reserve an economy car and once you get there the rate to upgrade is cheaper. Sometimes they will even give you an upgrade for free if there are no economy cars left.
Waikiki Sand Villa Hotel is a two minute walk to Waikiki Beach. Its only a 2 1/2 star hotel but for the price "under $100 per night" How much did you plan to spend? Ocean view will start at $200 go up from there. Generally Waikki beach hotels are older but look for one that has a recent remodel if newness is a concern.
I agree with don't spend the money on a room when you'll jsut use it for sleeping. Spend the money on tours, day trips, food. I've used the
Power Pass to get the best deals on a bunch of activities. It all depends on what you're looking to do. You might want to check out viators
Oahu Ideas. In general I always recommend booking tours and things to do in advance that way you spend your day having fun not figuring out what to do.