Where to begin?
When I get Chinese takeout, I salvage the containers. The old-style cardboard ones, which can be unfolded into a serving dish, by the way, yield a piece of good wire for use in pinning parts. The newer-style plastic dishes, oblong, with a clear lid, make good storage boxes on the bench. The soup containers, well, I use them to freeze soup, though I do use one as a dust-free chamber when I dip a canopy in Future and need to let it sit and cure.
I save the red plastic coffee stirrers from Dunkin' Donuts. They are cruciform in cross-section (there's a pun) and they are good stock for scratchbuilding. I save the stirring straws, too, also for scratchbuilding.
And I collected a bunch of wooden coffee stirrers at work. They make great planks in 1/72, 1/48 or 1/35. They are just about the right size for US carrier deck planking in 1/48. And when I mentioned to the guy who brings our coffee (affectionately known as "Mister Coffee") that I use them for modeling, he gave me two boxes.
Also from coffee--we switched to a service that serves one-serving packets. The packets are packaged on plastic rails, for loading into a coffee machine. We detach the packets for single-shot use. I collect those rails. They resemble guard rails and work in 1/24 or 1/20. I'm using a bunch of them to depict pre-fabricated shelter material in a 1/20 Maschinen Krieger build. And the packets have a plastic neck that has a weird, cool shape. I save those, too.
I save the small styrofoam trays my grocery store uses to package fruit. They make convenient temporary trays for holding bits of things for whatever project I'm working on.
I save the cardboard pallets that bottles or cans of juice, or some brands of beer, use to package a case. They also make great trays for keeping everything from one project together.
I salvage wire from old electronic and digital equipment, that's an old, old scratchbuilder's tip. Radios, appliances--cut the cord off, if nothing else. Computer equipment? Crack the case and pull the little black doo-dads and bits off the boards, and save those for detailing avionics or other electrical equipment bays. An old mouse looks like a turret on a sci-fi fighting machine. And from printers and scanners, I get good pieces of plate glass.
Old matchsticks can be used for bits of milled wood in small scales.
Pieces of twine and rope yield fiber for making grass in a diorama.
Small rocks and fine gravel and dust from the gutter in front of my house, winds up in my diorama groundwork, as do bits of dried roots. I also have a large jar full of old tea leaves, which I fix to a piece of root with white glue, for small shrubs.
Speaking of jars--I eat a peanut butter and strawberry jam sandwich every day for lunch, have now for 30 years. I save the plastic peanut butter jars for storing things, on the modeling bench but also out in the garage--nails and screws, wire nuts, the fittings for my compresser. I use the glass jelly and jam jars for soaking pieces in Super Clean for stripping, or mineral spirits, or isopropyl.
I scrounge the little glass jelly jars like you find in gift boxes, or at restaurants, for small amounts of solvent on my work bench, for cleaning my brushes as I paint.
When I buy a new dress shirt (button-down Oxfords), they are usually packaged with a piece of clear plastic to keep the neck shaped, and a little vee-shaped piece to keep the collar shaped in the package. I save those for clear stock. I've made smash-molded windscreens from that plastic, as well as to have cut or punched pieces as needed.
Cable ties (zip ties) can be used to replicate ammo feed chutes. I have a small stash of those in various sizes.
Get orange juice in a carton? Does it have a plastic seal that you pull out via a ring? Nice little piece of round, domed plastic. Save it.
How about the threaded plastic cap? Save it.
Old ball-point pens? You get a nib, which looks like a projectile; a plastic tub; a spring, and the bits that make up the plunger. Soak the tube in isopropyl to clean it, and get a wash of ink, too.
Do you like Guinness in cans? Drink the beer, then cut the can open and salvage the plastic widget that held a shot of nitrogen to help make your foamy glass foamy. Wash it out, and you have a nice little sphere of plastic. I saw a guy make a targeting pod on a 1/35 helicopter out of one.
Dental floss containers can look like storage boxes or crates for sci-fi subjects. I saved a bunch of those, too.
There are probably some that I've forgotten. I'll stumble across them when I least expect to.