People toss out computers, printers, and other electronics like used diapers nowadays. They are full of small mechanical parts, springs gears and gizmos. The electronic parts can be used as well. Paint up one or some of the many cylindrical shaped electronic parts (capacitors, resistors, or whatever) and you have a tank for liquids or compressed gas (junkpile it or mount on a loco). Trasistors, diodes or ICs packaged in metal cans make great metal cans

The wire leads may be snipped flush or left on at plumbing.
If you want junk, and most of us need more junk on our layouts, take this 1:1 junk and rip out a bunch of parts that are smaller than a cubic inch, toss these parts in a shoebox, then throw the 1:1 junk back in the trash before any governing bodies (aka spouse) sees you.
This photo shows a printer part, the black plastic part with a toothed wheel that rolls the paper along. It's being used as a gizmo on the blacksmith shop floor. Could be painted "rust" and be quite at home in a junk pile. All I did to it was drybrush grey to "pop" the details and dullcoat it. I brightened the pic to show the detail better, so if it's dark and you viewed it before my last edit, try "refresh."