For free, fun summer activities in Brooklyn for 3-year-olds, there are plenty of options—both organized events and everyday destinations:

Splash Pads & Water Playgrounds

• Brooklyn Bridge Park – Pier 6 features a large sand park, water area, and climbing gyms. It's a favorite summer spot for toddlers to splash and explore2.
• Prospect Park’s Harmony Playground and Imagination Playground both have water features perfect for young children looking to cool off2.

Parks & Nature

• Prospect Park itself offers trails for nature walks and exploration—even toddlers enjoy wandering the less-crowded, wooded paths on the east side, playing pretend, and following winding trails2.
• Brooklyn Botanic Garden is free on Tuesdays and before noon on Saturdays, offering beautiful gardens and open spaces ideal for young children to explore2.

Free Events & Programs

• NYC Parks' "Kids in Motion" program provides free activities for young children at many Brooklyn playgrounds, including arts & crafts, games, sports, and water play3.
• Free outdoor movies are held at various parks, including Brooklyn Bridge Park, Fort Greene Park, Prospect Park, Red Hook, and more—a family-friendly way to enjoy an evening outside together4.

Pools and Cooling Centers

• NYC Parks Outdoor Pools are free and open to the public, with lifeguards and staff on duty. Pools often have child-friendly areas for little ones to splash safely1.

Additional Ideas

• Many Brooklyn libraries offer weekly storytimes and activity hours for toddlers during the summer, free of charge (based on general knowledge).
• Local recreation centers have free memberships for children, and often have playrooms, art spaces, or toddler-friendly schedule blocks1.

All of these spaces and activities are free, but double-check schedules for special events or changes related to holidays or weather. Most are drop-in friendly, perfect for spontaneous summer adventures with a 3-year-old.