I went as a graduation from college present and stayed with a native of Hawaii who went to college with me and his family who lived on a farm. It was nice to have a personal guide, but it was too bad my future wife wasn't there (I get greif for it, and now she's using it against me to go to Ireland).
Drive around the island on the Road to Hana. It takes you past waterfalls, and stuff (although when I went, it was very very dry in the midst of a drought.).
Go up the mountain Haleakala (highly recommended and free except for gas). Nice drive, and there are some spectactular views on top (go in the morning time because you'll be able to see the sun rise over the ocean clouds).
Check out the Ulalena in Lahaina. I remember it being around 50 bucks a ticket, but it was pretty cool to watch. It's Hawaiian history to a dance thingy. I'm nut much up on this sort of stuff, but it was really cool.
Walk around the valley of Io near Kahalui (where the airport is). Nice stroll along a raging creek/river and some nice historical stuff to look at.
You can also walk along the giant resorts on the east end of the island. They're pretty cool.
Finally, if you like snorkeling, you can take a trip to Molokini (although I'll admit I didn't do this) which is an island where they used to do weapons testing that can be seen from Maui and is about 10 minutes away from Maui by boat (I'm totally estimating).