Knott's Berry Farm (another amusement park) - close to Disneyland
Magic Mountain (another amusement park)
Hurricane Harbor (next to Magic Mountain--water park) - both of these places are north of Disneyland, about an hour and fifteen minutes or so (depending on traffic, of course)
The beach, obviously. If you go west, the nearest beach to Disneyland is Newport Beach, I think. You could easily spend a day driving up and down PCH--there's a bunch of beach towns throughout Los Angeles that are pretty neat, like Manhattan Beach, Redondo Beach, Venice Beach, and Santa Monica.
If you have time, you could also go south about an hour and a half to San Diego. Lots to do down there, such as Sea World, the San Diego Zoo, and the San Diego Wildlife Preserve (or something like that, can't remember the exact name offhand).
EDIT: Oh, if you're into camping or outdoorsy stuff, you could also head northeast to Big Bear Lake, about an hour and a half east of Los Angeles.