First: What you should or are at least likely to already know about, and what I'll skip.
-Universal Studios Hollywood. Duh again.
-Six Flags' Magic Mountain (most coasters in one park on earth) and Six Flags Discovery Kingdom
-Cedar Fair's California's Great America (originally a twin to the Great America park in Illinois) and Knott's Berry Farm
-SeaWorld San Diego & Aquatica San Diego
-San Diego Zoo and Wildlife Park
-Santa Monica Pier/Pacific Park
-Santa Cruz Beach Boardwalk (multi-time IAAPA best seaside park winner)
You might be thinking, "Jesus, that's a huge list. Is there anything else to discuss?" Yes. Oh, very much yes. So much so, I seriously debating splitting this list up.
FROM SEA TO SHINING SEA
When Walt planned out the location of Disneyland way back in the early 50s, he ruled out an oceanfront location as beaches tended to bring out an "element" that he found to be negative to the purpose of running a theme park. What that element is; you can interpret on your own. Ultimately, he went to Orange County, and history was made. Meanwhile, California still had plenty of other such parks that either already existed or were constructed later. Pacific Ocean Park in Santa Monica and The Pike in Long Beach were perhaps the most famous, but there were many, many more. For a vast array of reasons, only two such parks still remain today.
I covered Santa Cruz Beach Boardwalk in my preface; it is clearly the class of the field and easily one of the two-three best such places in the US. Lesser known is Belmont Park, located just a stone's throw from SeaWorld San Diego. There's not a ton here in terms of rides; a new Tilt-A-Whirl, some small Moser flats, and a wood coaster. But the Giant Dipper is an important wood coaster. One of the great stories of coaster preservation, it was slated for demolition in the 1980s and had a grassroots campaign to save it that succeeded. Today, the "Pepto-Bismol Pink" ride still thrills guests as it has for most of the last 90 years.
BUD HAD A DREAM TOO
Historians of the amusement park industry have long recognized the importance of Wendell "Bud" Hurlbut. His Calico Mine Train is considered to be one of the first truly "immersive" attractions in the modern theme park space. As a concessionaire for Knott's, he operated in a different universe from the modern theme park expectation of everything being controlled under a single entity, and was responsible for creating the most impressive legacy rides and attractions for the Knott Family to boost his own income. While this style of park was exceedingly common 70 years ago, it is on the fringes in the United States, and is primarily seen in Europe. The Knott family ultimately took over the attractions, buying out Hurlbut. Often, these stories end there.
Hurlbut, however, ate, slept, breathed the amusement industry. And in 1976, he opened his own miniature version of Knott's Berry Farm in the city of Riverside, California. Castle Amusement Park is one of America's best small family parks; there's a Sally dark ride, a log flume, a couple of roller coasters (the Fabbri Mouse apparently isn't running, unfortunately), and unique for it's size level of design and care. Newer portions expanding the park aren't nearly as nice as the parts Bud himself worked on, but the fingerprints of someone who cared deeply about the customers of this industry are still all over Castle.
LOTS AND LOTS AND LOTS OF SMALL PARKS
Castle isn't the only impressive small park in California. Along with Disney and Knott's, the Valley is home to Adventure City. Through 2012, Adventure City's largest attraction was a classic Miler Mouse dating to the 50s. However, the decades of wear took a toll on the ride, and it was closed and replaced. Now occupying the spot is a unique for North America ride from Gerstlauer called Rewind Racers. Similar in style and forces exerted to the Vekoma Roller Skaters that populate the Disney and Universal Parks under various names, the most major difference is that this ride is not full circuit, but a shuttle which goes forwards and backwards.
In Ontario, one finds Scandia Amusement Park under the local airport flight path. There's some weirdness here in the small S&S Screaming Swing and drop tower before you even get to the big coaster, but that big coaster presents all the weird. The Scandia Screamer is the largest roller coaster ever constructed by US manufacturer Miler. Standing 90 feet tall and at 2,600 feet in length, it is a non-inverting steel ride which might best be described as "a poor man's traditional wood coaster". And like traditional wood coasters of the past, it has a start-stop pacing that may be jarring for some who are younger. This is not a ride that ever spends long periods of time at high speed flying through elements. However, like those old wood coasters, it offers brief moment of what one might call "ultraviolence": extreme vertical G forces followed by long low-banked turns, with hills increasing in number as the turns become tighter and less time is spent in them. For the serious connoisseurs, the Screamer is a ride that has a cult following.
Up north - didn't forget Northern California! If you spend time reading Cedar Fair financial statements, and don't even pretend, I know you don't, you would notice that they're contracted to operate a park that's not listed on the corporate site. Opened as Bonfante Gardens, the town of Gilroy has since taken over the now eponymous Gilroy Gardens and hired the theme park company to run the rides for them. Imagine a theme park/botanical garden that is like the old Disney California Adventure farm area on steroids. There's a garlic themed tea cup ride, mushroom shaped wave swinger, and a monorail through a greenhouse. How "Living With The Land" is that? There's also one of the last Arrow Mine Trains here, named Quicksilver Express. As it is only managed by Cedar Fair, your Platinum Pass isn't any good here, but that shouldn't alone be a reason not to go.
These three just scratch the surface: There's Funderland in Sacramento, Rotary Storyland in Fresno, and Pixieland in Concord.
TRAINS, TRAINS, TRAINS
If there's one thing that there is no shortage of in California, it is narrow gauge train rides. Disneyland and Knotts have their famous train rides, but there's also the trolleys of California Adventure and the funicular of Magic Mountain. Walt's old equipment runs at Griffith Park in LA too. But that's just the start of it.
When you talk about Knott's. the Ghost Town is what most people think of immediately. This is a wild west themed section featuring actual period buildings and some big time attractions. If you've been to Knott's, you know about Ghost Town. Funny thing about Ghost Town and the Calico Mine Train: There is really a Calico Ghost Town. Or was, until the Knott's bought and moved the buildings. But there still is, with replica buildings the Knotts family build to replace the originals. And you can go there. And go in parts of the old mine. And take a train ride. Just past Barstow on the way to Las Vegas, you can't miss it off I-15.
But wait: there's more! Off in Wine Country, the Sonoma TrainTown Railroad features a 4 mile train ride on their quarter scale railroad. In addition, there's a small carousel, an Eli Wheel, and a Miler junior coaster. Roaring Camp Railroads even offers to take you to a great amusement park - Santa Cruz Beach Boardwalk - from out of the redwood forests. The town of Pittsburg has a Herschell metal carousel and a Chance train. And of course, Balboa Park in San Diego has a train of their own, along with a great classic carousel with a working ring arm.
If you like your fake commuter rail to be elevated, the California State Fairgrounds feature a UM built monorail (similar to Hersheypark's) to take passengers all over the grounds. There's also a sky ride and permanent giant slide that stay put here all year, and on top of that, a piece of Disney memorabilia with the original "CALIFORNIA" sign from the entrance of Disney's California Adventure.
OF AN AQUATIC NATURE
Cedar Fair, Six Flags, Merlin, and SeaWorld all operate water parks in the state of California. And yet, it is Golfsplash Roseville that is home to the first water coaster in the state and the much of the most advanced slide tech California has to offer. Raging Waters in San Dimas brought a hydromagnetic water coaster to Southern California in 2015. Unfortunately, liability insurance seems to have brought death to all the older concrete style slides, leaving very little that's unique in this genre within the state. Mustang Waterpark in Arroyo Grande seems to be the one possible deviation, with a classic hillside design for their slides, though now made of fiberglass.
There's much more to discuss by the water: Over at San Francisco's Pier 41, Merlin recently opened one of their permanent haunts from the "Dungeon" brand. A small drop tower will be featured inside. Over by Long Beach, one of the two the last remaining of California's two classic electromechanical games can be found at Looff's Lite-A-Line. The Playland-Not-At-The-Beach Museum features the other one, a Fascination setup, along with pinball tables, miniature circus. Laughing Sal (proto-animatronic popular in the early 20s century), and all sorts of other historic amusement related material.
WILDLIFE, AND SOME RIDES
Aside from San Diego Zoo, there's a number of other zoos in the state with permanent rides, including:
Carousel and Train at San Francisco Zoo
Small kiddieland with kiddie coaster and wood carousel at Happy Hollow Park Zoo
Small kiddieland with kiddie coaster and skyride at Oakland Zoo
Carousels at Los Angeles Zoo and Sacramento Zoo