When you live in Hawaii, you always find that you have friends coming through the islands that want you to give them a tour and show them a bit that may be they would not otherwise see or places to eat that they probably would have never found if you had not taken them there. It's those small aspects that can change a great vacation or trip to the islands - into an even better one.
Quick Tips:
Rule of thumb for Oahu - if you cross the canal and depart from Waikiki, prices do tend to fall. Many never get further than Ala Moana Mall, but to see Oahu, you have to leave Waikiki - even though you can find so many things there.
Do not miss going to Chinatown for lunch or dinner one day while here. Also, the first Friday of every month is First Friday with many of the shops and art galleries and the like in and around the area that includes part of Chinatown staying open. If you are here - do not miss it.
As for souvenirs, you can find what you want, and then some, at the Aloha Stadium flea market on Wednesday, Saturday and Sunday. It will save you money, and it's a pretty fun time. Just try to go early in the morning (9 or 10am) before the sun starts blazing down.
As far as beaches go - Sandy Beach and White Plains are two great ones. Just know that when it's a bit rough - Sandy beach has a pull reach close to the beach break - though when it is 1 to 2 foot waves - it is the perfect beach.
Best Way To Get Around:
In and around Waikiki the trolley runs to many places, to include out to Aloha Tower and Ala Moana. City buses are a cheap way to get around the whole island. If you get a rental car, know that parking is always costly here downtown and in Waikiki. Rush Hour is on from 6am till 8am coming into the city from the other side of Pearl Harbor. It starts at about 3:30pm going the other way. Best bet is to be going the opposite way of traffic with a little bit of planning. An accident here can cause major gridlock, and it's just something you get used to. But do go over and explore the North Shore if you have a rental car - and the beaches there and on the eastern side of the island.