Also fun, 1-2 times a year the Spallation Neutron Source[1] is shut down and the tunnels are open for tours. Was happy to have had the opportunity to check that out when I was working there, although I think my favorite high-science tour is still the National Ignition Facility[2]. Going inside of that places was awesome. Thinking back now, LLNL had a number of great tours actually, HEAF[3] was pretty cool as well.

As far as I know none of those are open to the general public though; you have to work there. Someone I was dating at the time went on the public's tour of NIF, which really only involved them standing in the lobby. Ironically, she was a nuclear physicist and I do environmental and energy economics-y things, so I was completely unqualified to know anything about what I was seeing and she didn't even get a physicist as her tour guide.

[1] https://neutrons.ornl.gov/sns

[2] https://lasers.llnl.gov/about/what-is-nif

[3] https://wci.llnl.gov/facilities/heaf

I should, they do fascinating work.