In Waikiki Budget Accomodations are really easy to find the trick is to get the closest to the beach when getting really cheap accomodtions in Waikiki Hawaii. The best bet is to try the outrigger chain of hotels sometimes by going through Roberts of Hawaii you can get some outstanding deals
One of the best places to look for Waikiki budget accomodation is on the internet. the closer to the arrival date that you are planning to come and you should be looking at the vacation package deals that offer hotel and airfares as the airfares will probably eat you up if you try to book them last minute. Waikiki hotels run all year round at 80% or better on the occupancy so it is not a real good place to go and negotiate on the spot the big expensive hotels will normally have some sort of business rates but you are talking $200 a night ++ for the hotels in that range.
Waikiki beach covers about 5 Kilometers basically from Downtown Honolulu to the Diamond Head beach area at Kapialani Park at the zoo. The best areas in that stretch to look for the best Waikiki Beach budget accomodations are near the Hilton Hawaiian Village, near Fort Derrusy and in the city section of Waikiki heading towards the mountains from the Sheraton - Royal Hawaiian Hotel for long term stays your best bet is to look in this area closer to the Ala Wai Canal.
For the most part you will not really need a rental car once you get to the hotel schedule your car trips for the begriming or end of the holiday in Waikiki and walk the rest of the time. there are car rental places very close to all the hotels in Waikiki book a ride with the hotel shuttles to or from the airport.