Disneyland's second park, which opened in 2001 just yards away from its classic original park, might have started life as a misguided expansion, but it has found its feet and is worth visiting on its own. Its biggest blockbuster is the billion-dollar Cars Land, especially its fantastic Radiator Springs Racers ride, but other attractions hold their own: Guardians of the Galaxy—Mission: Breakout! is a successful rock-and-roll re-theming of the old Tower of Terror, the looping Incredicoaster speeds over the addictive Toy Story Midway Mania interactive ride and around an atmospheric lagoon where the World of Color water-and-light show prances nightly. This is also the park where you'll find the Disney favorite Soarin' (a simulated hang glider flight that kids love), a 70-minute live stage version of Frozen, and (new in mid-2020), an area themed to Marvel's Avengers.
Unlike Disneyland, Disney California Adventure serves alcohol nearly everywhere; good spots for that are the outdoor Sonoma Terrace, the lagoonside Lamp Light Lounge, and the upscale Carthay Circle Restaurant. If Disneyland is the park for nostalgia, DCA is the park for kicking back and taking things in.
If you must choose between this park and Disneyland, there's no substitute for the latter. But adding it to a visit is worthwhile—to do that, you need a "Park Hopper" ticket that costs about $55 more than a one-day, one-park ticket. And because it's about 100 steps from Disneyland, it's easy to switch back and forth between parks all day. They share the same parking structure.