Omiyage - from Manhattan back home to Hawaii
For some in the islands, omiyage can be viewed as "somewhat" of a tradition in some households or a family tradition (so to speak). That being said, any suggestions of what edible type of "souvenir or memento" unique to NY that I can bring back to Hawaii. This generally would be for the office and close family members. Just something rather small.
Are you checking bags? Are you comfortable putting liquids in your suitcase?
I'd think about black and where cookies, maple candy, maybe some hard pretzels from the farmers markets, or some babka. Pickles if you're willing to take the risk. It's such a long flight back to Hawaii that I think something like bagels would be stale by the time they get to the recipients.
Jam, honey, chocolate, and coffee from NYC are probably not necessary unless you have a special interest in them or get something really special like the Mast Brothers black truffle bar or hot garlic pepper jam from the Greenmarket. (Went to Maui earlier this year, the jams and coffee were amazing at the farmers markets; am looking into Oahu and it sounds like your farmers markets are amazing!)
What can you get through Hawaii agricultural inspection? That should be a consideration.
I think you might be OK getting some smoked fish at Barney Greengrass or Russ & Daughters if you can get it packed really, really well so that it doesn't smell at all, won't come out of its container, and will survive in your checked baggage.
I'm looking at Kathryn's suggestions, and I think maple syrup is much less fragile than maple candy, providing that it's packed very carefully if it's in glass (e.g., insulate the bottles by putting each in one or more different bags, separate them from each other so they can't hit each other, and put lots of clothes around them). Maple candy is susceptible to being smashed or melting, while I believe maple syrup is probably OK as long as the container doesn't smash, but if I'm wrong, someone will correct me. I haven't been lately, but last I checked, there was an excellent vendor from Vermont at the north end of the Farmer's Market in Union Square, toward the west side. My preference is for Grade B, the industrial strength grade! It won't be cheap, but it will be less expensive than you'd pay in Hawaii, and it would be a really considerate gift!
Babka is a good idea. There are various places you can get it, including Zabar's. It's sealed. It can go stale but would probably be OK for a couple of days, and it comes in several varieties (for example, poppy and chocolate). If you happen to be in the East Village, you could consider stopping at Moishe's and getting some mandel bread, which if it's fresh that day (ask), will survive better than a softer thing like rugelach, strudel, or black & whites.
You wanted to know when the Union Square market is open:
Open Monday, Wednesday, Friday, and Saturday
8:00 a.m. - 6:00 p.m.
Word to the wise: Try your best to get there by no later than 2 or 3, lest vendors start running out. But you'll probably still be able to get maple syrup or candy later in the day, I imagine. Also, though, not all vendors are there all 3 days, and you'll probably have the largest selection on Saturday. The map at the link I gave you will show you which vendors will be there on each day.