Lahaina, situated on the western tip of the island, is the largest town on the island and the most obvious destination for a day excursion. Many of the popular snorkel and SCUBA tours leave from Lahaina Bay and Lahaina is home to the most diverse food options on the island. Old Lahaina has some charming historic structures and a beautiful, giant banyan tree as well as lots of tourist shops, and the tourists that flock to them. The old buildings are interesting, but for a more personal experience of the town's history, visit the graveyard adjoining the Maria Lanakila Catholic Church on Wainee St.
PA'IA is the the town hub of Maui's North Coast, and is a beach bum's paradise. Although much smaller than Lahaina, its laid-back vibe feels more authentic to the island, and its shops are more mom-and-pop than of the chain variety. Pa'ia is also home to some spiritual locations like the Paia Stuba and a Buddhist temple; the place does have a certain energy to it. Have fun poking around (or just laying around on the beach) but don't leave town before eating at Flatbread Company.
MAKAWAO is a jewel of a town tucked away up in the hills that has become an artisans' haven. Several galleries and shops sell the locals' works, there's a small open-air market (where we had a custom beaded bracelet made for $15 based on the design of a $50 one we saw in a shop across the street), and the glass blowing-studio is a fun visit. The old-timey buildings and chickens bopping around create an altogether transportive atmosphere, so a trip to upcountry to walk around Makawao is really your best bet if you've geared more toward the quirky but authentic village feel.