Here in the USA for a while there, it seemed like credit card companies were sending me unsolicited mail on a pretty regular basis trying to get me to sign up for their cards, which included (fake) plastic credit cards with "Your Name Here" and a fake number printed on them.
I've saved dozens and dozens of them, and use them for pretty much the same generic stuff I'd use plasticard for - quick-and-easy bases, scratch-built shields and weapon blades, the sides of scratch-built crates, etc.
They're pretty much free - waste not, want not.
At the grocery, onions are often sold in these bags of plastic netting - I've saved the netting for years to turn into that camoflauge netting stuff that gets draped over artillary and the like, and I've also once used it for fishing nets on a scale fishing boat model, and even scratch-build model ship rigging (the sort of stuff that looks like rope ladders or nets, bracing the masts....) Again, totally free - why let it go to waste?
The scratch-built ships I used to build back in the '80s were made from "trash" treasures: cereal box cardboard decks and hulls, soda straw cannon and masts, paper napkin sails, etc.
Another one of my scratch-built model projects from the 1980s was an inexpensive 1/700 scale Nimitz-class aircraft carrier thrift-store find, missing the aircraft and a few other bits, which I souped up into a "20 minutes into the future" concept by using bits of sprue and cereal box cardboard to fashion the fuselages and wings of some tiny, tiny stealth fighters (or at least, the best public guesses of what stealth fighters might look like at the time!) I did some other work with the base model as well, such as using cardboard to build interior hangars with, and bits of spare model parts to make little scratch-built deck vehicles (fuel trucks, fire trucks, tractors, missile-loaders, forklifts, etc.) Lots of "trash" got recycled into that project!
As old-school plastic scale model builders know, those plastic sprues/runners that Wargames Atlantic figures are packaged on have thousands of uses for scratch-building stuff. For example, if you hold a length of sprue carefully over a candle flame while pulling the ends away from each other as the plastic softens and melts, it'll stretch into a long, thin strand that makes great antennae/aerials for scale model tanks. Here's a link to a YouTuber who loves coming up with new uses for sprues: (link) - his channel contains lots of that sort of thing, including building entire vehicles and buildings from sprues. You've probably got tons of sprues sitting around in this hobby, why not put them to use?
Spare miniature "bitz": anyone who has been in this hobby long enough has surely learned to keep all the leftover bitz stashed away in carefully-organized bitz-bins, waiting for a rainy day to find a use: extra heads, arms, weapons, shields, etc. Many of us end up with more bitz than we'll ever practically use in a lifetime! Find inventive ways to put them to use: severed heads for your barbarian horde, loose arms to embed in your trench diorama, Wargames Atlantic Classic Fantasy skeleton arms and skulls to clip into loose bones to decorate bases and add to dungeon dressing, weapons to clip from hands and cut in half as broken swords for dungeon tiles, shields to hang from dungeon walls, etc. Also, you can save some of those weapon bits for repairs to other gaming miniaures (I find that these extra weapons and shields, and replacement heads and arms, work great for dressing up Reaper Bones figures with droopy spears and other troubles, such as the relatively blank and featureless faces found on some of the earlier Bones miniatures. I tend to find a lot of Wizkids gaming mini sculpts to be painfully bland-looking, and often Wargames Atlantic bits are the perfect thing to breath life into a dull or poorly-molded Wizkids mini!) Think outside the box for some of those bits, too: spear shafts can be trimmed into sword grips, bows can be repurposed into horns and claws, etc. Unless you are gaming in a strictly historical setting, nothing is stopping you from experimenting with mixing genres, either: for that weird and seemingly distinctly '80s sci-fi/fantasy mashup vibe, go ahead and give that Dark Ages irish warrior one of those spare laser rifles and a leftover WWII German soldier's helmet from your bits bin - what would John Carter, He-Man, Luke Skywalker, or Yor (hunter from the future!) do?
Not really "trash", but: Don't be afraid to mix-and-match scales, either - I've saved some of the spare missiles and rocket-launchers from an inexpensive 1/72 attack helicopter kit to turn into some sci-fi weapons for some of my 28mm sci-fi guys: I figure it won't take very much trouble to scratch-build some sentry robot rocket launchers and other heavy weapons from this stuff! Cheap 1/72 scale historical Roman, barbarian, and and other infantry are just about the right height for 28mm gnomes, gremlins, or sprites. Similarly, dollar-store "green army man" figures are troll- and ogre-sized compared to 28mm figures, if you find some figures suitable for giant monsters. Those '70s and '80s era Tim Mee Galaxy Laser Team toys include some great "giant" sized lobster/turtle/alien-things, droids, and other characters that might fit the bill, and there was a Tim Mee fantasy action figure set as well that included "cave man" figures that make great quick-and-dirty cheap ogres. If you have some of these Tim Mee figures hanging around from childhood, this is a great opportunity to put them to use - and Tim Mee recently re-released both of these sets (along with their classic Green Army Men figures) in all their original, classic, vintage style we remember them for!
Cheap toys in general are a nice source of stuff to work with on 28mm gaming projects - you might need to add some weathering or other interesting detail and paint over some garish colours, but toy aisles and bargain bins, thrift stores, flea markets, and garage/yard/boot sales are some great places to find some outgrown toy vehicles, buildings, and action figures that can find new life as gaming minis. (For example, I've seen at least one or two YouTubers who have gotten a lot of mileage out of turning cheap superhero toy action figures into fantasy terrain statues....)
And, a heads-up: I've been able to find second-hand Wizkids "Hero-CLix" 28mm pre-painted random-package superhero figures being sold in bulk quantity for astonishingly low prices - I recently got a bulk lot of these for something like US$ 30 for something like 70 or 80 minis. The paint jobs are dreadful, the sculpting occasionally dodgy, and three or four of the figures were damaged, but all of this is easily fixed with some of your spare bitz from WGA figures, a little paint, and a little Wacky Gloo. One of the fun things about traditional superhero characters is that they are a product of pulp literature, which set a lot of modern sci-fi and fantasy genre conventions, without really being bound to modern genre conventions - that is, many of these minis actually translate pretty well to either sci-fi or fantasy characters with a simple weapon swap at most (along with a good paint job.) And, not a few of them work equally well for either fantasy or sci-fi characters. The lot I'd gotten also contained almost a dozen ninja characters (along with at least one complete set of ninja turtles!), who will make great fantasy RPG assassins and the like. I think I might be able to make use of about 1 in 4 of the minis I got, and the rest I'm giving away as Christmas stocking-stuffers. I'm guessing these HeroClix figures are on sale so cheap right now, because nobody else wants them - thrifty 28mm gamers who don't mind sorting through a grab-bag of this sort looking for a few to give a good make-over to should consider tracking some of these figures down and giving them a good home!
Styrofoam packaging: When I was a kid in the '70s or early '80s, my parents - running on a working-class budget in a thrifty '70s economy, ended up being forced to buy their first colour TV after the black-and-white set broke. I got the large, oddly-shaped styrofoam packaging - spacers and the lot - with a package of green army men and a fistfull of plastic astronauts and cowboys and indians from a neighbor's garage sale for Christmas.
I don't think I'd ever seen this sort of styrofoam packaging before, and was pretty confused - "What is it???" My father replied: "You figure it out. Looks to me like a moon base for your astronauts. Or, maybe it's a barracks for your army men, or a town for your cowboys and indians. What do you think it is?" Those big dumb chunks of styrofoam were maybe the best "toy" my parents ever gave me, short of the occasional shoebox: I used them for moon bases, castles, trenches, all sorts of things for the next couple years, until the stuff had seen so much use and abuse, my mother threw it out. I dare say, I spent more time playing with the styrofoam packaging "trash" than I ever spent watching the TV it was packaged in. So, that's my story of how I got a couple giant chunks of glorified styrofoam peanuts for Christmas in the 1970s! :D
Anyway, all sorts of stuff gets packaged with sheets and blocks and spacers made from styrofoam, which can be turned into terrain with a little creativity! YouTube is full of tutorials for how to use foam for gaming terrain projects - usually the crafters buy construction-grade foam sheets, but why do that, when there's lots of this packaging floating around for free??? (The stuff doesn't react well with solvents, as a warning: spraypaint and certain glues will melt the stuff down into nothing in an instant.... Try brushing on acrylic paint, and using hot glue or PVA glue, for anything other than disastrous results.)