Cedar Creek Falls from the San Diego Country Estates trailhead at Thornbush Road. This is on the improved trail, there are no difficulties on the trail if it wasn't for the elevation change and the temperatures.
For the waterfall area you need a $6 PERMIT (good for up to 5 people) from the recreation.gov website.
Personally, I would NOT recommend this hike between June and October at all, and in November, April and May only on cooler, overcast days. There is NO shade at all on the way down to San Diego River except for some chaparral and it gets HOT out there in the back country. Plenty of warning signs remind of the dangers.