The island is full of shopping malls, so you will be spoilt for choice when it comes to shopping. You can buy international brands as well as local garments like aloha shirts and skirts, as well as a number of locally made accessories and trinkets. There are waterfront marketplaces and flea markets if you wish to go shopping without going away from the beaches. There are also some themed contemporary retail malls and galleries that you should check out while in Oahu.
Gambling is illegal and the laws against it are pretty strict, so refrain from engaging in any such activities to avoid problems.
If you do not feel like doing much while in Oahu, you can simply head to the beach, lounge around next to the sparkling ocean, enjoy local food or a nice drink, and watch the sunset on the island. All the shopping and tourist areas are well connected by public transport.