This is a Colorado no brainer but, the Longmont Sugar Mill is a cool spot to explore. its also a good beginner location in my opinion. easy to get in, chill spot. Id recommend going during the day because there is a decent amount of holes in the floor that you could easily fall threw.

click to view

click to view

click to view

Good to hear it's open again! Last I heard (a year ago) it was sealed up pretty tight with a lot of barbed wire, so I'm glad that's changed.

It's interesting to see how similar it looks to the Sugar factory I visited up in Idaho. Same situation with the floors.

Any cool office space left there, or all pure metal and stone factory?

Just went here a week ago, had a blast exploring the two largest buildings closest to the railroad tracks (excluding the silos unfortunately) and got to see the roofs of both. The massive furnace room has graffiti on the outside of the smokestack on the roof, I can't imagine how people felt walking over to that...
I'd avoid the southernmost rooms, someone seems to be living there.
To answer Mossybread, there are some empty rooms with lab tables and cabinets/desks but they've been thoroughly wrecked. Photos attatched:

click to view

click to view

click to view

click to view