Although Maui is just a small island, there different areas are very different. Where are you staying? What city? If you are staying anywhere near Lahaina I can give you a plethora of things to do
Take some surfing lessons. That was one of the most fun things I did while there. Also, ask around for food - don't get caught up in the chain places around the island. Oh and food is extra $$ there (at least I thought).
Don't go with your shirt off too much without sunblock and don't be fooled by "shade" if you drive around part of the island.
There was a small mall off of Route 31 where I ran into a celebrity - you may want to go down there since it looked like there were a lot of private homes (South/West of the island).