If you are into Sushi, check out Sansei in Kihei. Great food and great ambiance; price is also very fair considering the quality of Sushi you are getting. Felix is also not bad if you just want fried stuff (fish, calamari, etc).
As for Lahaina, check out Aloha mixed plate, awesome beach front view, great food and drinks; price is also incredibly cheap all things considered. Note that you also get free music (next to old lahaina luau).
However, if you want to really experience what the locals eat, check out Sam Sato (order the dry noodle and yakitori) in Wailuku.
Have fun on your trip. Maui is the best!
Make sure you ask the locals about the Red Sand Beach near Hana. Not black but red! Keep on asking!

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