Just spent 6 months on the windward side (east) of Oahu, but did spend 2 months on the North Shore a couple years ago.
Paalaa Kai Mini-Mart - They have some quick food ready to go. Love their Spicy Shrimp Rolls. Plenty of shrimp inside and plenty of sauce where they are not dry. Also tried their fried oysters that had this great mustard sauce that went with it.
Waialua General Store - Good poke and once a week they cook a steak plate in their parking lot that is very good.
Foodland - If you just want some reasonable poke its very good. Typically I would get the Maui Onion.
Ted's Bakery - I thought they had the best Garlic Shrimp. Big plump shrimp. Their Chicken Katsu is also good.
Haleiwa Joes' - Have only been to their Kaneohe location. Their prime rib is good. It is big enough for two. Their other stuff is meh. Their other steaks are horrible if you are there too late for the prime rib.
Duke's - A must go. We went the last week we were there. Prime Rib was great. Pasta special was great. Everything was great. Great atmosphere.
Hula Grill - Same owner as Duke's I believe, it is right above it.
Leonard's - Malasadas (Portuguese donuts) Their main location usually has a long line. We go to one of their truck's around. I like their cinnamon sugar ones with no filling.
Ulalani's Shave Ice - I am not a snow cone guy but this is really good. There is one with long lines on North Shore called Matsumoto, but I found it wasn't worth going to.
Mountain Magic Shave Ice - probably as good as Ulalani's
I love Loco Mocos, probably had them at 7 or 8 different places. None of them really stood out as on another level. You can't really go wrong ordering on a menu.
Beaches are cool on North Shore to sit and watch surfers and a sunset. Sunset Beach, Ehukai (Pipeline). But we used to go to Aweoweo Beach Park every afternoon just to relax and get in the water. It doesn't have the big waves coming in and you can do a little snorkeling if you want. Not a whole lot of people. Kailua Beach on the windward side is probably my favorite beach to just enjoy the beauty and relax, get in the water, other scenery.