For a good lunch, ask some of the local shop owners for a good place to get "plate lunch". It's semi-fast food, which is usually some rice, potato salad, and a meat (chicken katsu and kalua pig are two of my favorites). These are usually mom-and-pop type estabishments, and each place usually has its own twist on the lunch. At the last place I had plate lunch, they had available a slightly bitter hot sauce made by the owner's mother. It was fantastic on the shredded pork. The food won't set you back much, and is a great way to take in the local flavor.
Also be sure to try shave ice during a walk through town. It's very touristy, but on a hot day it's delicious (especially with ice cream at the bottom).
Sorry I can't recommend any haute cuisine in Maui. It seems like every Maui fine-dining restaurant at which we've dined tried to do too much with the food. (Strange constructions that must be disassembled before eating, too-thick macadamia nut crust on a subtle piece of fish, etc.) Perhaps most tourists like that kind of experience? I'm not sure. I'm hoping others here have better suggestions for the fine-dining route.