Do:
Go to Maui Mall in Kahului (same town as the airport) and go to a small shop in middle of the mall near Subway called Guri Guri, and get yourself a 5 scoop mix of Guri Guri (sherbet type dessert in strawberry and pineapple flavors).
Make the drive to Hana (east Maui); this is an all day trip ... pack a lunch. Unfortunately, you can no longer drive all the way around the mountain (section closed off due to an unstable rock face), but oh well.
Hike the trails at Iao Valley.
Pay attention to your surroundings and those around you when walking around, particularly on Front Street in Lahaina. Tourists tend to be taking in the sights and are not as cautious when approached by people with bad intentions. Random assaults do occur by certain groups of locals in the area who like to pick on tourists. Although statistically it is a "rare" occurrence, if it does happen it can be a vacation ruiner. That having been said, don't stare at groups of locals as most of them are good folks, and staring at them will just exacerbate the problem; just be aware of them.
Don't:
Go shopping before going to the beach or hiking. Do not think that putting your things in the trunk or locking them in your car will keep them safe. HIGH incidence of vehicle break ins here. Think about going shopping AFTER you are done with your hike/snorkelling/activity for the day.
Leave your things unattended on the beach when going for a swim. Try to always have someone watching your things. HIGH incidence of thefts from beaches.
Leave your hotel room unlocked. Make sure all doors and windows, including upper floor lanai/patio doors are closed and locked. Keep valuables locked in the safe. HIGH incidence of burglaries.
.......
Sorry to seem like a downer with the Don't list, but I work in private security (hotel, community, and now golf resort) here, and I encounter thefts and such a lot. Tourists tend to be "surprised" that thefts and things happen "here". Unfortunately, the tourist industry tends to downplay the whole high rate of theft thing (for obvious reasons). I would rather you be aware and prepared than another theft statistic.
Despite my words of caution, the vast majority of visitors to the island come and enjoy their stay without incident, and I hope your visit will be the same. Awareness is key.
Anyways, have fun and enjoy your stay here!!
Victor