You have to have a car while on Maui, so make sure to reserve that ASAP, and then watch for prices to drop.
There is lots to do in Waikiki that doesn't cost much. Sit on the beach, take surfing lessons, take catamaran rides out along the coast, walk down to the zoo, hike up Diamond Head (do it in the morning so it is cool). Visit Chinatown in Honolulu.
On Oahu, you can take buses around the island if you want, it is not a bad way to see things. Stop at Shark's Cove for snorkeling on the North Shore. Stop at Kua'Aina Burger in Haleiwa for lunch, and have a shave ice at Matsumoto's in Haleiwa for desert.
Remember that parking in Waikiki is $20-25/day.
You really should try to see Pearl Harbor if you have never been there before.
Picnics on the beach are one way to save a little money. You can get cheap eats at a grocery store or even at the ubiquitous ABC stores.