Those shrimp shacks in the Kahuku area are awesome! The garlicky flavor is really tasty and perfect with the steamed rice. There's a few of these places very close to each other, probably the most famous is Giovanni's that you mentioned. I also prefer Romy's but the last few times that we visited the prawn's were somewhat of a disappointment. Too overcooked and not enough for the price paid (or worse, they were sold out). The regular shrimp plate is delicious. We usually order it to go and have a picnic at a nearby beach.
A good alternative on the North side of the island is the 'World Famous Yellow Shrimp Shack':
They have a variety of things on the menu than the other shrimp shacks:
The best is the pan fried shrimp plate with a side order of fresh Kahuku grilled corn on the cob:
I've tried their New Zealand mussels, they were pretty good:
But if you're driving around the North Shore area, you have to get a slice of Haupia pie from Ted's Bakery: