Get the car. Seriously.
Waikiki is fine. But the island is 597 square miles, and the stuff worth flying 11 hours for is scattered around it. A compact rental from the airport runs about $55–70 per day in shoulder season if you book three weeks out; walk-up rates at the Waikiki desks hit $120. Parking at most hotels is $40–50 a night, so consider a condo in Kaimuki or Kakaako with a free stall. TheBus is $3 one-way and reliable, but a single North Shore round trip eats four hours of your day.
Leonard's Bakery, Kapahulu
Ten minutes from Waikiki, on Kapahulu Avenue, and the line out the door is mostly locals. Malasadas are $2.10 each; get the original and the custard. Go before 9 a.m. or after 3 p.m. to skip the queue. This is the single best $6 you'll spend on Oahu, and it beats every $22 acai bowl on the beach.
Sharks Cove, North Shore
About 50 minutes from Waikiki on Kamehameha Highway. In summer the water is flat and clear — snorkeling here is better than Hanauma Bay and it's free, with no reservation system. In winter the surf is dangerous; don't get in. Parking is a small lot that fills by 9 a.m. Bring reef-safe sunscreen and water shoes; the entry is rocky.
Helena's Hawaiian Food, Kalihi
A James Beard American Classic winner in a strip-mall-looking building on North School Street. The pipikaula short ribs and luau squid are the order. A full plate runs $18–24. It's closed Saturday through Monday, so plan around it. This is the plate lunch that explains Hawaiian food to people who thought they didn't like it.
Lanikai Pillbox Hike, Kailua
40 minutes from Waikiki over the Pali. The trail is short — under a mile — but steep and dusty, with two WWII pillboxes at the top and a view of the Mokulua Islands that justifies the whole trip. Free street parking on Aalapapa Drive, but you'll walk a few blocks. Go at sunrise. Wear real shoes; people do this in flip-flops and regret it.
Kailua Beach over Lanikai Beach
Everyone photographs Lanikai, but parking there is a nightmare of no-parking signs and angry residents. Kailua Beach Park, five minutes away, has a real lot, showers, and the same powder sand. Free. Rent a kayak for about $45 for two hours and paddle to the Mokes if the wind is calm.
The budget math
Three days with a car: rental $180, gas $35, parking $0 if your condo includes it. Add $60 for food you'd have spent anyway. That's roughly $215 to see the island instead of the pool. Book the car the day you book the flight — Oahu rentals sell out in peak summer and Christmas, and the price only goes up.