This is your basic campsite. It's only open when there's not snow on the ground. I live at sea level so I don't sleep wonderfully at this kind of altitude, but that isn't the fault of the campground. It's quiet and on a clear night you can see a bazillion stars.
You have to do the Methuselah hike. It's short and there's not a lot of elevation gain but I did it on the first day at altitude so it was more challenging than expected. We were fine, but we passed a lot of complainers. It's worth listening to the ranger's talks at 11 and 2pm.
It's worth dragging your stuff up a small path to get away from the crowds. You can surf and mountain bike here. Our tent was even visited by some people on horse back!
I'm not much for car camping but if you want to get a big group together and sleep outside Cachuma has a ton of nice group spots. There are picnic tables, fire pits, bbq pit and even electricity to charge your phone.
There is a general store, a disc golf course, and a mountain biking trail. You can't swim in the lake (which is dumb, they say it's because it's drinking water but you can put a boat in and fish and I'm sure boats and fish have as many germs as people) but there is a pool. You can also rent kayaks and other water related toys.
The main draw back is that you're all over every neighboring campsite and the bathrooms were soooo, so, so disgusting. There was used toilet paper all over the floor, one broken sink, backed up toilets, ect ect. They cleaned them pretty often but blink and they were gross again. I would have much preferred to pee on the ground but since there's no bushes between sites that option wasn't available. My husband ended up hopping on his bike and riding to a porta potty that was more out of the way and way less gross.
If you like the feeling of having an entire island to yourself and don't mind hiking in a bit this is the trip for you. You access the island via boat. If you're packing for this trip you need to keep your hydration bladder and your camp stove fuel separate, the boat operators will make you unpack and repack if you do not. It takes about 3 hours to get there. We saw whales and dolphins on the way out and back, but our crossing was windy and choppy and more then a few people were blowing chunks off the back of the boat.
You get to the dock and everyone pitches in to get the gear off the boat and up the stairs. Then you get to start your walk to the camp site. It's about 1.5 miles from the dock and very pretty. Some people brought a rolling cooler but I do not recommend that. The path has a lot of rocks and soft sand and pulling a cooler over that looked like a huge nightmare. We brought a yeti soft sided cooler (the smallest one) which worked out perfectly and fit inside the fox box easily. The strap around your neck makes it super easy to carry.
The campsites each have a wind shelter where you set up your tent. The wind is not a joke. When we came back from hiking more than one tent was blown over. Low profile, use your steaks and we even tied ours to the shelter to keep it stable. There is water at the campsite (and indoor flushing toilets!) but it's pretty hard to drink. I brought a few lemons to squeeze in it but they don't cover the strong mineral taste. It's fine for rehydrating your food, but you probably want to bring in drinking water.
The hiking is awesome, but steep. Most of the trails are longer than you think because the distance is from the dock not the camp ground. Lobo Canyon which is a popular hike - and worth it - is way longer than you think because the distance listed is from the dock to the trail-head. It's another 2 miles to the beach from there.
There are Torrey Pines trees, which are rare. Island foxes, which are SO CUTE and a bunch of large birds.
At certain times of the year you can camp on some parts of the beach but I wouldn't try that on your first trip out there. I also recommend against bringing out a surfboard unless you take your own boat out there and can drop anchor where you chose. It's 8 miles to the nearest surf break from the dock and the waves aren't always worth it.