Brooklyn has two free Saturday options with matching hours and very different settings. Both are scheduled for October 17 from 11 a.m. to 3 p.m., so the practical choice may be which neighborhood is easier to reach.
Waterfront at Pier 6.
Brooklyn Bridge Park's Harvest Festival takes over Pier 6. The park lists live music, arts and crafts, educational activities, a mini pumpkin patch, face painting and family activities. It is a free walk-up event; food and beverages are available for purchase, and the park says the event may change because of weather or other conditions.
Bluegrass at the Naval Cemetery.
The Greenway Stomp is listed at Greenway Botanical Garden, 61 Williamsburg Street West, at the Naval Cemetery. The organizer describes food, activities for kids and adults, bluegrass music and a children's costume parade. It is also free.
These are separate events, not a single cross-borough festival. Check the park's Harvest Festival page and the Greenway's event calendar for any last-minute schedule or weather updates.