You'll be due north of me, I'm in East Northport - the town with no sense of direction! I'm not a foodie but a few thoughts:
Northport is a great harbor town, check it out even if what brings you here doesn't take you to Main St. There are plenty of restaurants on Main St. but some are cash only.
From a foodie perspective my understanding is that https://maronicuisine.com/ is the place to be but they don't open until 4PM and probably not a drop-in place.
Feed and Grain is a good option for outdoor seating https://www.thefeedandgrain.com/
Not on Main St but well regarded is Robke's. https://robkesofnorthport.com/ . Cash only. The place is tiny. Constantly posting pix on facebook with sports figures eating there.
Another option is Huntington which is a town or two to the west of Northport and has tons of restaurants (that afaik all take credit cards). I really like Besito https://www.besitomexican.com/huntingtonny
And of course since you'll be on Long Island stop at a local bagel shop and buy a dozen. Fun fact: although bagels go stale really quick they can be revived by a microwave (or frozen)
Hope this helps, safe travels.