I typically recommend the California Science Center in Los Angeles. Admission is free but it's $10 to park in their lot. It's perhaps not a great museum, but they do a lot of things reasonably well and in one location. They have a small aquarium, several aircraft/spacecraft on display, and assorted stuff you'd expect in a science museum. Aquarium of the Pacific is probably a better aquarium. There are better aviation museum and better natural history museums. This place however puts all of that in one place.
The star attraction is the Space Shuttle Endeavour. That requires a timed ticket for a nominal fee. It was $2 when we visited last year. The timed ticket is included if you buy an IMAX movie ticket. You may need reservations on a weekend. We went on our return from DL on a Monday and getting tickets at the window wasn't an issue.