Castaic Lake Campground is basically a parking lot with a few amenities throw in. There's a very nice park on the opposite side of the lagoon from the campground and I'm sure in the summer months it's a busy place. You can sail, picnic, and cycle around the spacious grounds.
We were camped there during the week and no one was in the A loop with us. There were a couple of campers in the other loops, but basically, we had the place to ourselves. I took the bikes out and rode up the road to Lake Castaic, the campground is located on Castaic Lagoon. It's a great ride up to the lake and the views are worth the uphill ride.