Scott and I had done this hike a few years ago and we were completely alone at the waterfall. You have to climb a rope down a steep hill to get there, but it’s not that difficult. This time we went on a holiday weekend and we were still alone at the waterfall. I hate to even post about this because it’s such a well kept secret. First off, the drive getting there is worth the trip, you drive about 35 miles on Angeles Crest Highway through the mountains to the Buckhorn Campground. Finding the waterfall is a bit difficult, but you should be fine finding it if you follow these directions precisely: http://www.hikespeak.com/trails/cooper-canyon-fall/ Be sure you pay $5 for an Adventure Pass on your way into the park otherwise you will get a ticket.
You could spend all day there. Afterwards, if you are hungry you can stop at a biker bar and watch live music at Newcombs Ranch. http://www.yelp.com/biz/newcombs-ranch-restaurant-and-bar-la-canada
This is really the perfect day. The hike itself takes about 45 mins each way to the canyon and it’s not that strenuous.