Check for reservations 24-48 hours before desired date, most cancellations happen within this time period
Watch the sunrise/sunset at Haleakala National Park. Reservations and tickets are needed for the sunrise (recreation.gov) 60 days in advance. No reservation is needed for sunsets. This is a national park so you will need to pay to get in, but it's worth it!
Hike in Iao Valley!
Get Guri Guri (frozen dessert)
Road to Hana
There are a ton of local companies that will take you around Maui or even off island to go snorkel. Highly recommend checking it out!
There are various companies that teach surfing to beginners and rent out boards. If you are staying at a hotel, they may have their own surf school or have recommendations.