I was stationed there for 3 years. Oahu is very nice but get out of Honolulu to find cheaper things to do. Hanama Bay is the best snorkling (beautiful), North Shore is a lot of fun but I definitely don't suggest trying to surf there. Kailua is a very nice town on the other side. It's over by Kaneohe Bay. (Jack from Lost hangs out at Kailua beach and lives in Kailua) The climb to the top of Diamond Head isn't too difficult and the view from atop is awesome. If you're on Oahu go to the "swap meet" at Aloha Stadium. Great prices for suveniers (sp?) and there's tons of stuff there.
Maui is beautiful as well. Cool for whale watching. The Big Island is fun if you go to the volcano. (stinks though) Kauai is the most beautiful island. This is more the "movie type" Hawaii. Very plush and green.