We just got back from land vacation to Maui.
Check out local tour van to Hana you may have enough time and the scenery is unbelievable.
Trip to Haleakela is shorter and very unusual scenery, but only go if the mountain top looks clear.
Lahaina is a wonderful little town filled with restaurants and shops and galleries. Also you can do a historical walking tour of Lahaina. Lahaina Restaurants I can recommend are Bubba Gump Shrimp, Cheeseburger (in paradise), Kimo's all with wonderful views. For cheap lunch Aloha Mixed Plate, right on the water, open air dining, plastic plates and paper cups, great plate lunch. Lahaina Cannery Mall has free entertainment usually 11 am to 1 pm and there's a food court for an inexpensive lunch.
For shopping Whalers mall in Kaanapali and upscale shopping in the mall in Wailea.
If you like ice cream don't miss Lapperts (still missing their coconut/carmel mac nut ice cream).
Maui ocean center (aquarium) is wonderful and shows you what's in the waters around you without having to go snorkeling (for those of us who get motion sickness).
If in Kaanapali for dinner Hula Grill is wonderful. For the best fish on the island it's Mama's Fish House (set in a coconut grove on the ocean near Paia) great for lunch or dinner. Near Mama's is Hookipa where you can watch the windsurfers and see the blow hole, very windy there at certain times of the year.