Headen Park in Thornleigh provides two reasons for visiting, the same reasons you'll find at most sportsgrounds; the oval and the playground.
The oval here is used for two different sports, depending on the time. You'll find it ready for cricket in summer, and rugby in winter. There are changerooms for the players, seats and toilets for the spectators and even lights if it's nighttime.
As mentioned, the park also has a playground, with an equipment set that offers a slide, nets to climb and things to spin around on. This area seems to be well-maintained, though not regularly upgraded (the equipment set is getting a bit old). There is a fence around the park, but it's not any kind that will prevent kids from leaving the space.
Headen Park conveniently has its own car park, which is accessed off Sinclair Avenue.