Because Maui is so small, basically every adventure on the island will take no more than a day to get to and complete. Here is my list of some epic day trips that most tourists probably bypass.
Ferry ride to Lanai: How many day trips do you know of that involve getting in a boat and traversing the Pacific Ocean? Wake up early and head to the Lahaina harbor to catch the first ferry over to Lanai. The total ride will take about 45 minutes, and if you are lucky, a school of spinner dolphins will escort your boat into the Lanai harbor. From the harbor, take a five minute walk over to Manele Bay (once voted one of the most beautiful beaches in the world). Enjoy some great snorkeling in the bay and then make your way over to the rental car company to take out your very own 4×4 Jeep. Once converted from pedestrian to off-roading adventurer, take pleasure in exploring dirt roads that lead to various scenic points on the island. Once your day on Lanai is complete, board the ferry back to Maui for sunset! Read about my personal Lanai trip itinerary in 2011 here.
Road Trip around the West Maui Mountains: I am sure you have heard about the road to Hana and how scary it can be. You may have even purchased a shirt that reads “I survived the road to Hana.” The road to Hana will look like the landing strip of an international airport when compared to parts of the trip around the West Maui Mountains. The road basically hugs the meandering coastline, and is wide enough to accommodate one car. For the safest journey, I would recommend starting the drive on the Lahaina side, since you will be hugging the mountain on any sort of pullouts required (you will face scenarios where you or an oncoming car will have to reverse to a safe pullout). The views along this side of the island are very beautiful and there is even a fun art gallery along the way that provides a perfect pit stop from the nervy drive.
Coastal Walk in Kihei: This trip idea should only take you a morning or afternoon, but it may wear you out for the remainder of the day. The coastline between Ma’alaea and Kihei is pretty much unobstructed. You can walk about five or six miles without ever leaving the pleasure of waves crashing up on the shore. Get a friend to drop you off in Ma’alaea and walk back to your car parked in Kihei. Search for shells along the walk, wave to the three or four people you will pass along the way, and simple enjoy having a beautiful Maui coastline all to yourself. The below map show the coastline I am talking about.