From the parking area at the south end of the parcel, it's possible to hike the perimeter of the parcel and explore quite a few of the park highlights. Head north, toward the center of the park, passing Reard House and the Heritage Garden on your left. (To your right just past the Heritage Garden, the treehouse is visible; you'll pass it again on the way back).
Continue on Main Street before arriving at an intersection, where you'll turn left onto a graveled path, walking west toward the edge of the park boundary. When you arrive at the boundary, turn north (right). You're now on the appropriately named Westside Trail. Continue to a Y junction and take the right fork (the left one will eventually lead you to the north exist of this parcel and into Big Rock Park - North).
The right fork passes through forest with a sweet little pond on your left, Owl Pond. Soon you'll arrive at another Y junction. Stay to the right, and now you're heading towards the east side of the parcel on the Busy Bee Trail. At the east edge of the parcel, turn right and hike south along the edge of the park. There are four trails branching off to your right as you hike south. Each of them can reconnect you to the main street trail in the center of the park if you wish to do that.
Otherwise continue south, passing a final junction to the right that leads to the treehouse (which is only open occasionally and requires reservations. Continue south until reaching the edge of the park, where you can turn right and head back to the parking area.