Trying to find 1980s YA novel — NYC scavenger hunt, blonde boy, “Serendipity” necklace

I’m trying to find a YA paperback I read in the 1980s. It was a summer adventure/first‑romance type book, probably from Scholastic Point, Avon Flare, Bantam Starfire, or a similar imprint.
Here’s what I remember:
Girl protagonist, around 15
Her little brother (maybe 8–11) goes with her
They take a trip to New York City
A blonde teenage boy (16–17), the son of family friends, comes along
The kids have to do a scavenger hunt or clue‑based adventure through NYC
At first the kids think the trip will be boring
They are given rules for the scavenger hunt and the word serendipity pops up and the girl doesn't know what that word means but the the blonde teenage boy does
The little brother is chaotic but becomes important to the story
A sweet first romance develops between the girl and the blonde boy
At the end, the boy gives her a necklace that says “Serendipity”
The title has with the word Serendipity
Cover art (very vivid memory):Illustrated 1980s YA style. The blonde boy has his arm around the girl. The girl is wearing an I ❤️ New York shirt and she’s holding a mini Empire State Building souvenir. Her little brother is standing on a gray concrete half‑wall. He’s wearing a Statue of Liberty foam crown and he’s holding an ice cream cone like a torch. It was definitely a mid‑1980s YA paperback, not a picture book or adult romance.
Possibly titled something like Summer of Serendipity, A Serendipity Summer, or similar.
Does anyone recognize this?

Remember Me to Harold Square by Paula Danziger. Several covers exist; one matches your description.

This is your book! Thgere's a sequel called Thames Doesn't Rhyme with James. Love them both!