Road to Hana, rent a car if you arent already day rates are cheap, and drive the road to hana, its a gorgeous ride and you can get out along the way see waterfalls and such. IF you make it to the end there is a black sand beach...but it takes ALL day
You can also ride up haleakala crater(sp), you can see EVERYTHING from up there
there is also a vinyard/winery there, and its nice if you like wine tasting, they have some interesting wine and tour
also this guy has a BEAUTIFUL lavender farm, people get married there, its gorgeous