Disney's California Adventure is right across from Disneyland and is worth the park-hopper pass. And if you're in town for long, the best deal would be an annual pass. I think California Adventure complements Disneyland very well in that it has more intense, turbulent rides than Disneyland. I loved the water rafting ride, the California Screamin' rollercoaster, and the Hollywood Tower of Terror. And we went on a random Wednesday in October, so the lines were less than a 5-minute wait and allowed us to ride all the good rides multiple times. Going during fall on a weekday is probably the best time to go in terms of lines and weather. And if you go on an off-peak day, you can definitely cover both parks if you start early in the morning.