Since cake vendors can only do so many weddings in a day, with all the delivery, setup and driving in traffic, you want to be sure that you schedule with your baker early so you can get exactly what you want. Pick your favorite cake with the best icing and make sure it will be at your wedding where you want it.
Try something unique like a flavored cake and icing that go together. This way you don't have to have the traditional white cake with vanilla icing or chocolate cake with chocolate icing. Even if you just mix it up by having a Hawaiian-styled white cake with a citrus flavored icing it would be a serious improvement over traditional cakes.
Cake tastings are usually available if you call in advance. This is a great chance to taste a variety of cakes and icings to better understand what you're buying for your wedding. Otherwise you may not be happy when your cake isn't what you expected and it's already been served to your guests. However, is cake really ever bad?
There are so mnay style of wedding cakes now, with squares, circles, small cakes, big cakes, etc. You can even have fully customized cakes make to look like anything you desire. They cost more, but they're really cool. Think about the flowers, cake toppers, and other decor that could dress up your cake as well. There are just so many ways to make it uniquely your own.
If you're having a lot of guests, try smaller versions of wedding cakes or maybe cupcakes. You can purchase several small and beautiful cakes to place at each table instead of or in conjunction with the large cake. You can then choose several different flavors, designs, and looks. Speak with the cake designers to weave all the looks together, and make it flow.
Have you seen this new idea where you create a phony cake that looks good for your guests and then when you serve them cake it's actually another cake their eating? The large (phony) cake is hauled into the kitchen and then disposed while the real cake is served to your guests. It's an interesting approach to making your wedding cake unique.
Since you have to have a cake at your wedding; it's just unacceptable to expect your guests to skip cake; you can always go after the smaller cakes, sheet cakes, home-made cakes (although this can be very stressful for the baker) and ultimately you want your guests to have somethign to talk about. Whatever you decide, make sure you're happy with it. Getting married in Hawaii should bring the local ambiance into your wedding anyway, but the little additions will make it so much better.