The California Science Center is a free museum with all sorts of different exhibitions to explore. From space travel to life sciences, every room has educational and interactive fun. It's also huge so there is plenty to look at. While it is free, they do take donations if you are able.
A lot of it caters to kids but you still learn new things when you go! There are different rooms with various earth climates with animals and an aquarium as well as massive air and space crafts you can look at and even go into. They show all types of movies that you will have to pay for, but are fun to watch.
The Science Center is also near the Natural History Museum and is accessible through Metro travel. Though, if you need to drive, the parking is free.