I camped in Mustang during the last weekend of September. It was located right next to the water park but they had already closed for the season. The campsites were pretty close to each other and unfortunately we were stuck between two different groups that did not obey the quiet hours. Not a huge deal at night since they were quiet by midnight or so but one of the groups was also loud with kids early in the morning as well. This didn't make the best camping experience but we tried to make the best of it.
The lake was within walking distance but since the water level is so low, you had to walk quite a way out. There are some decent hiking trails you can go on right from the campground. We took the Escondido trail from camp that took us to the top of a ridge and had numerous switchbacks. Great views from the top.
This is a great place to go if you like riding off-road vehicles in the sand dunes. There are no assigned campsites here and there are 1000 reservable "spots" on the CA State Parks website. Camping is cheap at $10/night with a $7.99 reservation fee. During the summer, this place books up quick. We had to reserve a month in advance. Since there are no assigned spots, you can choose who you camp next to (or don't camp next to). We had a great spot at the end of the dunes but unfortunately some disrespectful neighbors brought in a whole band (complete with a tuba, accordion and loud speaker). They played until 11:30pm when the rangers came to shut them down. Not the most ideal neighbors but we had a good time riding in the dunes.