The San Gabriel Mountains National Monument is a great place to get away from the grind of everyday life in SoCal. You might not have time for a long weekend in the Sierras but you can easily make it here for a quick overnight or a relaxing weekend.
The Horse Flats Campground is located 29 miles up Highway 2 from Interstate 210. You will turn left at the Santa Clara Divide Road(this intersection is also known as 3 Points). 3 miles down the road you will turn left to enter the campground. I have camped here a few times and its always been pretty quiet. The sites are $12 per night and are first come first serve. Make sure to bring water, they do not have running water at this site. I have never seen horses here, but there are corrals near a few of the spots. There are vault toilets(which were pretty well maintained every time I've been). When planning your trip make sure there aren't any fire restrictions (https://www.fs.usda.gov/recare...)
The Silver Moccasin Trial is a 53 mile trail that runs next to the campgrounds (near site 11).