1. Haleakala National Park: As the island's "main natural attraction," the park is home to the world's largest dormant volcano. The crater itself is 3,000 feet deep and seven and a half miles long.Frommer's: House of the Sun: Haleakala National Park
2. Hana: The "beautiful" and "lush" coastal town is "rich in Hawaiian history." Frommer's suggests spending two to three days to experience the town's "beauty and serenity."Frommer's: The End of the Road: Heavenly Hana
3. Snorkeling: This activity is "a must" according to Fodor's, and labeled "the main attraction in Maui" by Frommer's. The island's waters are full of "colorful" fish and "mesmerizing" views. Some of the best snorkeling spots include Kapalua Beach, Kamaole III Beach Park and Ulua Beach.Frommer's: Maui Snorkeling Fodor's: Top Maui Experiences
4. Helicopter tours: Many of Maui's most "dramatic" and "beautiful" natural sights, like the Wall of Tears, can only be viewed via helicopter.Frommer's: Maui Attractions Overview Fodor's: Helicopter Tours

Fodor's also has several "great 1-day itineraries" that detail the best outdoor activities to do on each shore of the island.Fodor's: Great 1-Day Itineraries