My favorite place on Maui is Pacific'O in Lahaina. Don't miss the tempura maki. The restaurant is right on the beach and sunsets are spectacular. It's a bit pricey, but the food is great.
There's also a good cheap plate lunch place on the north end of Lahaina called Aloha Mixed Plate. Leilani's and Hula Grill on Ka'anapali beach both have solid seafood.
Kula Lodge on the way to Haleakala is expensive for dinner, but much more reasonable for breakfast and lunch. Food is okay, views are spectacular.
The Paia Fish Market has relatively cheap and fresh seafood.
On Kauai, we had less luck, but we had a good meal at Duke's Canoe Club in Lihue. We had some really good shave ice with mac nut ice cream on the drive up to Hanalei, but I'm blanking on the name. I'll post if I come up with it in time.
Gorge on local seafood that is hard to find elsewhere, like opah, ono & opakapaka. It's wonderful stuff. Have a great time!