I'll throw out a couple inexpensive things
Hanauma Bay Take Kalaniana'ole Highway east from Waikiki for approximately 10 miles. Entrance to the preserve is on the right.
Currently OPEN from 6am - 6pm (winter - October thru March) and 6am - 7pm (summer - April thru September) Currently OPEN every Saturday from 6am - 10pm throughout the year Currently CLOSED every Tuesday
Cost:
$1 per car parking fee - large, paved parking lot - arrive early as the lot fills quickly.
$5 per person admission fee for 13 year old or older; Free for children under 13 years
Kawailoa Beach a beautiful sandy beach, you can learn to surf, pickup some good local food. Umm Umm Cheap!
If you want to see the tourist sights the best deal is to pickup a
Power Pass a 2 day powerpass for $115.00 and a three day for $150.00. It includes free admission to 25 top attractions in Honolulu like Dole Plantation Maze, Battleship Missouri Museum, Kualoa Ranch, USS Arizona/Pearl Harbor Tour, Sea Life Park, Iolani Palace, The Bishop Museum, Hawaiian Waters Adventure Park and lots of discounted products.