O'ahu is the main island, and is the island most people see when they come to Hawaii as this is where you'll find the metropolis Honolulu and of course the well-known Waikiki beach.
O'ahu has so many things to offer that you can easily spend a whole vacation just exploring this island, but if you have more than one week, I would recommend visiting another island, to see the variety of Hawaii's islands.
Things to do and places to see:
Where to stay:
On our first visit we stayed at the Hawaiian Seaside Hostel, a very nice hostel with nice staff. They arranged different activities among others a great BBQ night, movie nights and theme nights (when we were there we took part in a fantastic Halloween party!). The location is great, close to the beach and the shopping street but still not too noisy. The prices were similar to other american hostels, so around 30-35 dollars for a private room with shared toilet.
On our second visit we rented a small flat at the Kuhio Banyan, which was actually in the same area as the hostel. The rooms can take up to 4 people, but they are not too big so I recommend just to be 2. We payed around 90 dollars per night for the room, but I think the prices are very dependent on the season.
http://www.kuhiobanyanwaikiki.com/