I remember a couple of places that stood out.
One was an aeroplane parts manufacturer. Which meant really massive hangers where huge bits of wings (or whatever they had to make there) would be. I remember even feeling a bit woozy when I walked into an empty hanger. I guess my brain could not handle the fact that I saw a massive 'outdoor sized' space indoors. And because it was for aeroplane industry every bit and bolt had a huge stack of papers next to detailing every quality test and manufacturing spec in triplicate. So they had a lot of a machines and a lot of clicks.
Another interesting one was a decommissioned underground military base that consisted of hundreds of barracks to protect soldiers from a nuclear war. Now the base was sold and converted to business units, each former barrack housed a small start up or internet firm.
The only problem was finding the right customer there. Imagine really long underground hallways, all with concrete walls and identical metal doors and hardly a sign in sight. If you were lucky the company had a business card sized logo stuck on their door otherwise you had to go by the even smaller numbers written on the doors. To keep a long story short, you could easily get lost there and it was impossible to call somebody since there was no reception in what was literally a nuclear bunker.