I did an internship at Sandia this past summer, specifically on the HPC programs. Being a PhD student, the pay for that internship was excellent, and the cost of living in ABQ is low. I lived somewhere that I could bike to my office. My manager was excellent and a technical staff member and really understood how to manage other technical staff; no pointless meetings, hands-off, and tried at every turn to shield us from boring administrative non-sense.

The work was interesting, though my experience was very skewed as I was working on a solo project. I met weekly with my "mentor" to discuss where I was and if I needed support, but I was working nearly entirely solo. Like the parent comment here says, from what I learned the norm is that you work on many projects -- my mentor certainly was. Even in the department meetings it was clear that while we were unified under a general theme, each person in the department was working on their own, many projects.

My work was entirely unclassified, and my understanding is that most of the people in my department worked on projects with similar levels of classification. My office building looked like every other building and infosec and opsec requirements were pretty mild; wear your badge, don't photograph things, don't tell people any specifics about what you do.

I was offered to stay on as a full-time intern as part of the hiring pipeline and if it weren't for that it's in ABQ I would have strongly considered it -- the work was very interesting and also like the parent comment says, you are largely in control of what you do there. It's a lab first, not a defense weapons company, so research is the name of the game for the department I worked in.