Drive to Lahaina for snorkeling at either Molokini Crater, Honolua bay or Kapalua bay. Molokini Crater will require for you to take a tour, however the other bays are easily accessible by the beach but mostly swamped with people.
For daytime activities, walk down Front Street and you will find plenty of opportunities for shopping, eating, and exploring art galleries. Front Street also has a stretch of bars you can visit, but keep in mind that Maui really isn’t an island meant for nightlife. If you want to grab a great dinner with local Hawaiian dishes, try Lahaina Grill.