Hike through the Bamboo forest- On the the road towards Hana about 15 minutes past Paia.
Sun rise bike ride down Haleakala, the volcano in Maui - This one speaks for its self!
Snorkeling and Whale Watching trips - Highly recommend doing this, but be careful about which boat you choose to go on. Many of them offer highly discounted trips, but tend to be more like tourist conveyor belts that cram you on a boat with up to 130 other people and there's nothing special about those trips.
Paia Fish Market - located out in Paia- AMAZING fish tacos and the bonus of a cute little town to walk around after you eat. Is about half way to Hana. Definitely worth stopping by if you are headed that way.
Kimo's -You can get the best fish dishes OR the best Prime Rib you will ever eat. be ready to share with someone though because it is like 32 ounces. They also have wonderful Mai Tai's and other island drinks
Just lounge on the beach, body surf in the ocean, watch the maui locals surf, shopping right there at whalers village