Aside from the obvious, lying on the beach, there are many activities to do on the great island of Maui Hawaii. These are my favourite.
Snorkelling in Maui is absolutely amazing. The water is warm and calm and the coral reef is absolutely stunning. It is often possible to spot sea turtles.
2- Drive to the top of Haleakala Volcano
Driving to the top of Haleakala Volcano is absolutely amazing. The mountain is quiet high more than 3000 meters (10,000 feet) and the view from the crater is breathtaking. While climbing the weather change drastically two or 3 times due to the different altitudes. A great stop on the way there is the Vodka factory, the Goat Farm and the Lavender farm.
3- Dolphins and Whale Watching
A trip to Maui is not complete without a great Whale Watching tour (in season December to March). The amount of whales and dolphin that is possible to spot is stunning. Due to the fact that Whales go to Maui to give birth it is quite common to spot the baby whale swimming next to the mother.
Dolphins are permanent residents in the Hawaiian Islands and they are very easy to spot. Many tours offer a swimming with the Dolphins experience that I strongly recommend.
4- Walking to the Nakalele Blowhole
This site is at the far northern tip of the island. It requires a bit of a long walk in a very rugged terrain but it is totally worth it. The view from there is quite stunning, the rock formations are spectacular and the blow hole is really neat. Word of caution do not get too close to the blow hole, it is quite dangerous.
5- Taking lots and lots of photos!
If you are a photographer, you like photos or you simply own a cell phone with a good camera, Maui is a photographer paradise. Sunsets are incredible, the ocean is amazing and due to the fact that there is virtually no light pollution night photography is really amazing.