There are plenty of free magazines containing information for tourists in stands all around Waikiki.  These brochures offer tons of tours, discounts, and maps.  Also, you should check out the Go Hawaii site, which is Hawaii's official tourism site and is available in multiple languages (Chinese, Japanese, Korean, Spanish, German, and English).  It offers information on all the islands, and is helpful for planning any trip, whether simply in Oahu or a multi-island trip.  Be sure to order the Free Visitors Guide, click the link on their main page and they'll send it free just about anywhere. Connected with GoHawaii is another site just for travel tips: check it out.

 You should also check out the sites for the local papers, the Honolulu Advertiser and the Honolulu Star Bulletin.  They occasionally offer good tips for finding cheap (or even free!) events.

There is also the Hawaii Visitors and Convention Bureau, which administers the GoHawaii site.

Also check out the Hawaii Tourism Authority, which is funded through a tax on hotels and motels, and lists their mission statement as: "To strategically manage the growth of Hawaii’s visitor industry in a manner consistent with our economic goal, cultural values, preservation of natural resources and community interests."