There is a surprising amount of hiking available from the general campground area of the Park. A paved road leaves the Park Headquarters, goes through the campground, and continues up to the Vista Point parking area, which has a toilet. At the start of the Castle Dome/Root Creek trails, there is an old roadbed that angles down and provides a shortcut to the Pacific Crest Trail below.
From the east (freeway) end of the parking lot, you can walk over to the viewpoint, then backtrack a bit and head down a pleasant trail [shown at left] to a connection with an old road.
From here you can turn left (toward the freeway noise) and hike around the hill, ending up on the Pacific Crest Trail as it climbs up from the trailhead at the Soda Creek offramp on Interstate 5. This is the Kettlebelly Trail.
If you turn right instead, you can follow the old road all the way over to the road that heads down to the Park Headquarters and up to Bob's Hat Trail and beyond. There is a junction here with the upper end of the Milt Kenney trail.
Paralleling below the road is an old railroad grade that fades away in spots and is quite overgrown in others, but the large fills where it crosses canyons give it away. The Milt Kenney Trail [shown below] crosses this and ends up on the paved road near one of the campground units.