The four most interesting Hawaiian words I have learned so far:
Makai and Mauka: Asking for directions from a local here requires a change of reference in geography. While we, non-native Hawaiians, tend to think in terms of North, South, East, and West, the native Hawaiians do not. For them, everything is island-centric. So there are two important Hawaiian words you need to understand:
Makai means toward the sea.
Mauka means inland, upland, or toward the mountains, or if you're at sea it means toward the shore. In weather reports here, you will hear mention of "mauka showers." Indeed, on most days since I have arrived here, if one takes a gander mauka, one will see the peaks of the mountains hidden in clouds, and we have frequently found it raining on the the H-3 highway going across the middle of the island while it's bright and sunny everywhere else.
Hokupa'a: I took my boys to the Bishop Museum. They have an observatory and a planetarium there, and we attended a show in the planetarium on how the Polynesians sailed between Hawaii and Tahiti using the stars for navigation. One of their main reference stars was hokupa'a. Hokupa'a is Hawaiian for "stuck," and it's the Hawaiian name for the North Star (because it's stuck in the north sky). I thought that was pretty cool.
Le'ahi: I recently picked up a copy of O'ahu's Hidden History. One of the first things I learned from this book is the Hawaiian name for Diamond Head is Le'ahi, or "head of the tuna." I thought that made a lot of sense, because if you look at it from the Waikiki side, it really does look like the head of a tuna.