Fagan Park

This place is massive. By massive I mean 55 hectares. There is so much stuff here it is worth checking out their brochure/​map (pdf)
I have only been here a few times, and that was for the Hornsby Bushland Shire Festival which is on once a year around September. It is always a crowd pleaser. Heaps of food stalls, arts and craft, and shows for the kids makes the day pass pretty quick. This year my boys enjoyed the Sponge Bob Square Pants show the best.
While the $ 5 entrance fee (per car) on the day of the festival is a donation, I believe on any other day the park entrance will still cost you $ 5. Though for a picnic area this huge, with ponds and bridges, play areas and sand pits, gardens and paths… you will not lack for things to check out. It even has BBQ facilities. You might even want to bring along the push bikes for the kids and let them ride around.
Certainly worth checking out if you are in the area.