I lived in Hawaii, Oahu for most of a year. You'll love it. Here's some sage advice:
Treat locals with deep respect. They are talked down to by some tourists like they are servants or simpletons. They like being treated with dignity and not patronized. Hawaii is a state, but locals consider themselves a country really. They may not say it to you, but most locals feel they were taken over by the USA, against their will. They are told their Queen was kidnapped and coerced into agreeing to become part of the USA. They were always very nice to me. One of the locals told me it was because I treated them like normal people and respected that I was a guest visiting their islands. If locals give you mangos, it means they like you and it's a form of hospitality. Julian and I were hanging out watching the sunset on the west side of Oahu. That's a place they tell you not to go. But a Hawaiin family came and gave us mangos and everyone seemed almost delighted that we weren't afraid of being on the Tabu western side.
Groceries, especially milk and gasoline, etc. will be shockingly expensive. Generally, you can save by checking out the sales at the drugstores and Wal-mart for milk, bread things like tuna, soup, ramen noodles, etc. Some of the grocery stores will let you sign up for one of the scanner club cards you put on your key ring and you can get certain items cheaper each week if you show it to them. Be prepared for skads and skads of tacky mu mu's in the K-mart and Wal-mart stores. You can wear flip flops anywhere and there the locals call them slippers.
The north and east sides of the islands are generally the lush, tropical parts of the islands and the southwest and south tend to be hotter and more arid. On Oahu, the SW side is desert. I am guessing Maui is probably the same way. I know the biggest waves tend to be on the north side of the island in winter, after the first of the year. The waves on the north side of the island will be much bigger during winter and more dangerous to surf. The calmer waters, better for snorkling, tend to be on the more southern sides of the islands. Unless you can find a lagoon. Lagoons are cool because the waves are tiny and you can float sitting up in the salt water. Use sunscreen. You burn very, very quickly in the subtropical sun and be very miserable.
Hopefully the restaurants are better on Maui than they are on Oahu. You can't even get decent Chinese or Asian food there. Hawaiians love Spam. Not kidding. Be sure to get good advice on the restaurants. Sunsets are free and amazing. Going to the movies can be fun.
There aren't any seagulls in Hawaii because the chain of islands is the most remote place on earth. They are farther away from a continent than any other island. It gets dark earlier there and the sun comes up earlier. You will have no trouble getting up because you will lose to five to six hours from Eastern Standard time and three to four from Pacific Standard time. All that depends on whether or not there is daylight savings time going on the mainland.
If you go in winter, bring some zip up, hooded sweatshirts and pants and sneakers because it can get chilly, drizzly and windy. Also, the nights at any time can be cool and very windy.