We spent a week and a half on Oahu in June and LOVED our vacation! We rented a condo in Waikiki at the Ilikai and it was freshly renovated, very nice, and convenient. We ate breakfast in the condo (I recommend grocery shopping at Walmart) and found some reasonable places to eat in Waikiki most evenings. We enjoyed eating at a coffee shop that was within easy walking distance of our hotel. We also ate at the snack bar at the Hale Koa twice (very reasonable prices) and at a couple of different restaurants at the Ala Moana mall which was very close to our hotel. Everyone talks about how expensive the food is in Hawaii but honestly, we didn't find it any worse than most resort destinations, including WDW. We used the car rental place recommended in another post here (discounthawaiicarrental) and we paid less than $400 for a 7 passenger vehicle (Hyundai Santa Fe) and had the car during our entire trip. We purchased the GoOahu cards in advance and felt they were a good value for us but I did do a lot of planning in order to maximize the value of them for us. I bought the cards from Costco and received 4 days for the price of what the GoOahu website charges for just 3 days. We used the cards to take a 2 hour Catamaran sail, for admission to the Polynesian Cultural Center including the luau and the Breath of Ha show, for the jungle tours at Kualoa ranch for 4 of us (highly recommend) and for discounted horseback riding for 2 of us (also highly recommended), for admission to Waimea Falls (we loved swimming in the waterfall), for admission to the Pearl Harbor museums and audio tour headsets, for snorkeling at Hanuama Bay (loved this- highly recommend) and for the train ride and maze at the Dole Pineapple Plantation (this is the only activity I would not do again). We also hiked Diamond Head (amazing views) and Makapu'u Lighthouse, hit Turtle Beach and saw two sea turtles sunning on the beach (a highlight for my girls), ate at Giovanni's shrimp truck in Haleiwa and hit Morimoto's for the best shave ice I've ever had, spent some time lounging at the beach at Waikiki, ate fresh malsadas from Leonard's bakery, and hit the swap meet at the Aloha bowl for cheap souvenirs. Did I mention we had a fabulous time?
I hope this helps. Have fun planning!