We like SoMtn park as well (probably because we live in south Phx), many options, here's a couple.
Option 1. Very close to AZBrad123's suggestion, except, drive in the main entrance on south Central. Follow the signs up to Buena Vista overlook. Take the National Trail (east) down to Fat Man's Pass, Hidden Valley, Natural Tunnel and loop back west on the National Trail.
Option 2. Drive in the main entrance on south Central. Follow the signs all the way to the west end of the Alta Trail. Take the Alta Trail as far as you like then double back on the same trail. Or, complete the Alta Trail and loop back around on the Bajada and National Trails.
The Alta Trail is one our favorites because there are a lot less people, sometimes nobody
. The Natural Tunnel is far east as we ever go, from that point east to the 48th street park entrance there are waaaaay to many people.