Mostly tools worth stopping for.
3/4 drive ratchet
Stihl 16" chainsaw in it’s case.
Hammers
Also stopped for:
Ratchet tie downs,
4 bales of hay
blankets
coats
rope
rolls or wire. Never need to buy bailing wire again.
Other stuff I might could use but picky about actually picking it up after I see it up close.
Not highway, but an ordinary city street…two weeks ago, I found a bundle of party balloons, shiny pink mylar, one shaped like a “2” and another like a “1” (don’t know if the celebrant was 12 or 21), with regular round white balloons attached. It was floating, like a ghost, about 6 feet off the ground down the center of the street, with strings trailing below.
I pulled over my car and grabbed them and brought them home (first looking around to see if it looked like anyone was chasing them). It made for a nice decoration. And as it turns out, one of my daughters is just the right age.
Bungee cords!!! My family knows I will stop and pick up bungee cords from the road, so long as it’s not dangerous to do so. One can never have too many bungee cords.
Quite a few tools - recent additions a 6" pipe wrench and a pair of bolt cutters with a roll of solid copper ground wire. A large Igloo water can. Oh, and back in high school a huge cardboard box of drugs, looked like the leftovers from a drug store burglary. I didn’t see anything of obvious recreational value so I threw it back.
Life jackets. People pull their boat out of the water and forget to secure stuff.
Man, all I ever find are stray socks and the odd shoe now and then.
Hundreds of bungee cords, butt-loads of tools (usually always a 9/16" wrench or socket, but now and then a hammer or nice 10mm open-ended wrench), a camper shell that fits my wife’s Nissan and works just terrific after a little repair, an FJ40 hard-top that I made a bunch off of on Ebay, my neighbor’s smart-phone.
A kayak. It was laying in the middle of the east bound lane of I-70 a few miles east of Salinas, Utah. This was some 5-6 years ago.
I was going west and pulled over and waited for several minutes before walking back to where it was. I pulled it out of the highway (there’s not really a lot of traffic there) and hung around a bit more. Finally, figuring that if they knew they’d lost it they would have pulled over at once, I drug it back to my vehicle.
Still have it.
A wig. It was rolling like a tumbleweed down the shoulder of I-95 during a substantial snowstorm, but I caught it and brought it home with me.
Actually, it was mine and it had blown off when I got out of the car to clear the windshield.
Many years ago, when I was much less worldly, I was driving down the interstate. It was around a holiday weekend, and there was heavy traffic, moving at a fast clip. I saw a suitcase, apparently intact, in the grass median. There was no way I could safely get onto the shoulder and dash across two lanes to retrieve it, and I felt really bad about that, because, hey, these poor people lost their suitcase and what if somebody dishonest found it? A short distance up the road, I came upon a flurry of activity on the shoulder. There were about half a dozen police cars, marked and unmarked, pulled over with their lights flashing, and in the middle of all these police cars was a U-Haul. The back end was open and the contents were spread out along the shoulder. My thoughts, in rapid succession, were (a) those must be the people who lost the suitcase (b) it’s a good thing they got onto the shoulder before the rest of the stuff came out and © aw, man, I wish I could have gotten the suitcase because I’d be able to give it back to them.
So I went home and told my mom and my sister about it. They both just kind of looked at me like they were really wondering if I should be allowed out by myself, and then they told me that most probably the people in the U-Haul really, really would not have appreciated having the suitcase returned.
Oh…I said.
In my many decades of driving these roads, I have found enough stuff to fill many pickups. If I were to write a non-fiction book, I would start with one about these items.
Since the OP limited this to items found on highways. I will stick with those item that I found on limited access roads.
I have found at least three small tool boxes full of tools on separate occasions. The first one I literally ran over while riding down I-5 near Mt Shasta.
I had just been allowed back on the freeway by the CHP. They did not like the idea of me riding my Triumph Trophy over the snow covered hill without any traction devices when they were requiring snow chains. When I returned from MT Shasta City with nylon rope wrapped around my rear knobby tire they laughed at me & sent me back to try again. Yet they were kind enough to warn me that I had best just get a motel, as they were soon going to close the road for the night.
The next morning they opened the road with no restrictions. The snow plows had been working all night, but in the dark they did not see this tool box & ran it over pushing it into the snow with their rear dual tires. I bumped across the tool box while climbing the hill. It was plain to see what it was, so I stopped. After dodging semi trucks & snow plows, & using my tire irons to dig it out of the slow lane, (Very dangerous! do not do this), it came free. I strapped it onto the seat behind me & rode on.
The next one I watched fall out of the pickup in front of me. I was driving my service truck down I-84. A pickup in front of me had the tailgate down & was weaving all over the road. I suspected that the driver was “under the weather” as they say. In any case I could see many of his tools & this small tool box were loose in the bed. He did not pull over after many attempts to get him to do so. So I backed way off & let him go his way. I saw this tool box fall out. When I stopped to pick it up I noticed that many more of his tools were strewn all over the highway. I took the time to pick them all up. Three chainsaws, four axes, five gas cans, a handyman jack, some chokers, & several iron bars later, I drove on.
Well, this is enough typing for tonight. More later, maybe.
I forgot the sheet of paper story.
It was spring, and the weather radio kept going off with tornado warnings. I left for work a few minutes early that day. Most of my drive was along a rural highway, flatland fields with the occasional small wooded area. It was overcast but not storming, and I kept watching the sky, not that I’d have anywhere to go if something happened. I reached a certain spot in the road, a bit of an elevation with open land, and all of a sudden a giant hand reached out and tried to shove my car off the road. It came from the south, this huge gust of wind, the kind that has you fighting the steering wheel, and–that was it.
“Well, that was weird,” I thought, and I glanced at the clock. I remember this, because I remember it was just then the time I would normally leave for work. Then immediately up ahead, I saw debris on the road. I expected it to be normal storm debris, twigs and leaves. But it was trash, basically. Just stuff. Then I looked up, and illuminated against the grey sky, there was this single sheet of white paper, way up there, and it just floated down to the ground, very slowly. And I remember thinking, “I want that paper. I want to see that paper.” Somehow it was very important, because of the way it had come out of the sky. But there was no shoulder, and no way to safely pull over and run back to get it. So I went on.
I got to work to tornado sirens blaring. A co-worker pulled in beside me. My cell phone rang, which was strange. I only use it for emergencies, and the only people who actually have the number are my sister and my husband. I answered it, and it was my sister, saying, “Where are you? The weather radio is going nuts.” I told her I was at work, everything was alright, not to worry. Beside me, my co-worker was having the same conversation with her sister.
We went inside. (If you’re wondering why the tornado sirens hadn’t already made us run for the building, tornado sirens in the spring are just sort of background noise here, and besides we knew once we went in, security wouldn’t let us leave until the all-clear.) Later on I went to a patient’s room–this was a hospital–and he was watching the local news and remarked that there had been a small tornado in a nearby town. This was mentioned several times throughout the night, including the time it had happened.
Which was right about when the giant hand tried to shove me off the road. When I got home that night, I got out a detailed map and was able to pinpoint where I was on the road. I was in an almost straight line with the town that had gotten hit. It was a distance of about 5 miles, and that’s the direction the gust had come from.
I really wish I’d been able to get that piece of paper.
You sir, deserve a Major Award.
*Since I am no longer in a hurry, I would love that place. *
You gotta move to New Jersey. Well, western New Jersey, the rural part. Deer everywhere, jumping across the road, in your backyard, in your front yard.
We have that in rural Arkansas, with no gangsters. 
I found a Nintendo on the side of the road once when I was in high school, probably 18ish years ago. I found cords to one about two months after. It worked! I gave it to a family that had foster kids. They played it a lot.
Spilled Pepsi: a Pepsi delivery truck lost a lot of its contents out of the trailer somehow. There were plenty of bottles and loose Pepsi on the ground.
All I have found on the highways and by ways are a bunch of stupid idiots that dont know how to enter the highway ramp accelerating. Dead deer too
A pallet or two of newspapers that had fallen off a truck. And then come apart.
There was so much paper on the freeway it looked like it had snowed.
I volunteered for a few months on a highway cleanup program. I’ll just say this…One. Shoe. What is up with all the solo shoes being tossed out windows? It was never a pair, just one shoe. I am convinced this phenomenon is somehow closely related to the ‘one sock in the drier’ mystery. Let me know if anyone knows the answers because I sure don’t.