NASA Ames Research Center at Moffett Field
NASA Ames Research Center has an Visitor Center Museum that is free to the public and open Tuesdays through Fridays from 10am to 4pm, and Weekends from Noon to 4pm. They are closed on Mondays and Federal holidays.
Even though the museum looks small, my daughter and I spent the hour we had available there, and could easily have spent more time if we'd been able. It isn't that there is so very much to see, but many of the exhibits are interactive, and includes some games that she wanted to spend considerable time on. I eventually had to pull her away so we didn't miss our tour.
Fortunately, there is also a free website where they offer very similar games, if slightly simpler, to the ones she enjoyed at the NASA Visitor Center. It is at: http://www.scigames.org/
From their website, it sounds as if the exhibits in the Visitor Center change on a fairly regular basis. If you visit the NASA Ames Visitor Center web page you can also see a schedule of events and presentations that happen throughout the day so you can time your visit accordingly if there is something your family would be interested in.
The NASA Ames research Center also offers free school and homeschool group tours. That was the main reason we made the trip there. We just stopped to check out there Visitor Center since we made it to Mountain View so early, and I'm glad we did. The Visitor Center would have been closed at the end out our field trip, and it was well worth the time we spent there.
Although I did not set up the field trip myself, I was lead to understand that setting one up is more complex than other field trips might be. Moffett Field is a working NASA research center and they take security seriously. I believe the mom who organized our group had some hoops to jump through, and as a chaperone I had to make sure I had registered with our group appropriately, and had my identification all in order. Despite all of this, the trip was amazing and wonderful. When my youngest child is old enough, I would not hesitate to put in the work needed to set up a tour for her. And, on the plus side, it was all free.
The trip is aimed at kids in 4th through 6th grades. They get to do a lot of hands on guided lessons. They even did two mini 'missions' that involved assigned age-appropriate research tasks, and communicating their finding to the rest of their crew.
Information about how to set up a field trip is also available at the same link for the Visitor Center.