See GSGLA's list of activities!
Every February, the Los Angeles Orienteering Club puts on an annual Scout Orienteering Event with some training and two chances to run orienteering courses. However, they also give brief instructions before EVERY event in the LA area -- check out their web site.
The JPL Open House takes place on a Saturday and Sunday in mid-May. It's fun -- and it's FREE!
Not free, but a great experience -- the Disneyland Youth Education Series offers activities led and taught by a Disney facilitator for different age levels, and covering a number of different interest areas. Check it out!
In addition to normal tours, the Aquarium of the Pacific offers various classes.