On west coast beaches:
Miles of rope and fish nets, japanese fishing floats. Odd assortments of flotsam and jetsam and a surprising amount of heavier metal objects that wash in (God knows how because they certainly didn't float there.) Plus seafood if you understand how to fish and forage for it.

Northern forests:
Anything near a town, even a small town is likely vandalized or picked over too bad to help you much, but that means you are fairly close to town and help.
The problem is that with skidoos the kids can get 50 or 60 miles into the woods in the winter to wreck stuff like line shacks, construction and logging camps, rail sidings and their Jeep sheds.

Old mine camps. (be very careful because there might be open mine shafts in the area. If you fall down one you might not come out)

I have seen fairly recent mining camps that were simply abandoned when the mine ceased operations. They just shut the doors and walked away because they were not worth hauling out.
One included a fully equipped kitchen with pots, pans and the silverware. You would have been able to move right into the place if you needed shelter.
Older places are likely to have sheet metal, nails, wire, iron bars or pipes even if the buildings are gone.
Containers are common in their dumps. Tin and plastic was likely burned or is rusted and decayed to bad to be useful. You will find glass jars if the antique hunters did not beat you to it.

Old rail lines had tool sheds and often they simply left everything when they abandoned the line. There quite a few smaller rail spurs years ago. They are sometimes still marked on the topos.

Many of these mine, construction, rail and logging camps are noted on your topographic maps, but not all were. Keep an eye out for old roads.
Mines are all supposed to be noted on the geological and claims maps, but a lot of the older ones were never recorded. Again, watch out for old mine shafts and don't step into one. The worst are vent shafts because they are small enough for the bushes and moss to hide, but they might be hundreds of feet deep.
Think of falling down an abandoned well, same idea.

Portage points. Often you find useful stuff at the start or end of a portage trail. It was likely left there to help you, or the old trapper who used to travel that line, get over the portage.
Trappers often stashed canoes at the end of a portage to save themselves carrying it over the portage again. He would land his canoe and carry his stuff to the canoe at the other end of the portage, then paddle to the next portage and repeat. Reverse direction on the way back out.

Dam sites. There was likely a camp for it and a portage around it.

I am not suggesting you deliberately go out of your way to seek these types of places out just for scavenging.
If you have to travel it is much easier to walk down an old rail line or road than be breaking your own trail. You might be lucky enough to have an old road going your way.
That might be better than the materials you might find.
These places are often the reason those roads are there.