Stay on oahu if you want to do the touristy thing. waikiki, the north shore, lanikai beach. Hotels can be expensive bu there are cheap hotels in waikiki if you look hard enough. You can learn to scuba dive or snorkel at haunauma bay, dont know about certs though. If you're looking for naturous adventures spend more time in Maui. Hawaii is incredible, the people are very friendly and laid back, have fun.