You don't say which side of Maui you will be staying, but there is a nice quilt shop in Kihei (sort of south side of the Island) in the Azeka Shopping Center, it carries nice fabric and offers classes occasionally.
The Maui Quilt Shop
1280 South Kihei Road
Kihei, Maui HI 96753
http://www.mauiquiltshop.com
In Kaahumau Shopping Center (close to the airport), there is a shop called Sew Special that carries nice fabric, both Hawaiian and regular.
Another neat shop on the west side of the island is the Needlework Shop in Lahaina. They carry a lot of Hawaiian Applique supplies and material, if you call in advance, you can set up a free class to learn Hawaiian Applique when you buy a kit from the store.
136 Dickenson, Unit 3
Lahaina,HI. 96761
808-662-8554
There is a lot to do and see on Maui, but probably the best thing to do while there is nothing :D, just relax with a good book while listening to the birds chirping and waves crashing on the beach :)