Oahu offers plenty of delicious fruits and veggies. The island is also full of shops selling beautiful orchids. If you're heading to the US mainland, however, beware. You will need to declare all agricularal items before checking in to your flight and will go through a second inspection after passing through security.
Items which are allowed: leis (I brought mine home and dried it), packed coffee, items purchased after inspections. Some areas of the airport sell pineapples and orchids that have been checked and are allowable on planes.
The website below is from the State of Hawaii and offers further information on USDA regulations.
It is generally not allowed unless it is pre-approved fruit. On leaving the island you have to X-ray your bags for agricultural items. Then later on the little dog comes around to sniff your bag and take your papaya and mangos. So if you want to bring some fruit back, be picky about how you spend your money - or you might end up wasting it.
You are NOT allowed to take certain leis back to the mainland. If you are purchasing a unique lei somewhere, mention to the person you are buying it from that you will be taking it to the mainland. They will let you know which you can take and which you can't.
Don't be surprised if you see a gallon of orange juice for $10 or milk for $7. IF you don't believe me, look at the picture.