The Peters Canyon Regional Park in Tustin is one of the most popular hikes in Orange County, offering trails with challenges for all different levels. It's a gem.
The official public entrance is on the north side at 8548 E. Canyon View Ave. in Orange, but most hiking groups meet at the south entrance to avoid having to pay for parking. The south entrance is at the intersection of Peters Canyon Rd and Silverado Terrace in Tustin, though many GPS navigators don't do a very good job of getting you there based on this.
Note: when navigating, be sure to enter Tustin, as there's also a Peters Canyon Rd in Irvine.
But parking at the southern/back entrance is universally problematic, and many hikers become lost or frustrated (or both) when trying to join the group for the first time. I've been to three or four hikes now, and I'm still getting used to it.
This map shows the local streets around the southern trailhead, with details and pitfalls noted:
Peters Canyon Road is rather long, and there's no parking at all on the left side, and it's only allowed on the right side once you pass the reservoir access road. People do receive tickets here.
The residential neighborhoods on the left are gated communities, so there are no parking opportunities there either.
Peters Canyon Elementary School is on the corner of Pioneer Rd and Peters Canyon Rd, and though they have two parking lots, it's not clear they are open to the public (signs say "parking by permit only"). I know some hikers do park here, and if it's on an evening with no school events going on, there may be no harm. If you park here, just walk to the trailhead along the main road.
Further to the south is Cedar Grove Park (run by the City of Tustin), and there don't seem to be any prohibitions about parking here as long as it's not overnight. Though it's possible to walk along the main roads (Pioneer to Peters Canyon), you can also cut through the park and shave off some time.
It's about 3/4 mile from the Cedar Grove parking lot to the trailhead, so if you use a walking app, start it here. It counts, right? Be sure to allow sufficient time to park and get to the hike (especially the first time).
I understand that there is additional parking in the neighborhoods just north of the trailhead, but I haven't done those yet: I'll document them once I have.