Other than the stuff that's always in there... spare tire, tool kit, medical kit, we usually only carry luggage. When the kids were small we sometimes packed a cooler with drinks and sandwich stuff.
In the Winter I always try to put a D-handle plastic-blade scoop shovel back there. Most of the rest is always in there: jumper cables, super-capacitor pack jump starter, 12 volt tire inflator, 3 orange-flasher magnetic emergency lights (double as flash lights), a couple of microfiber rags, a pair of old "mud shoes." Spare, jack and tools came with the car. A few of the old plastic grocery bags. Remainder of a roll of duct tape.
I always have road flares, spare, hand cleaner, rags, etc. I also carry paper maps. Back in the day, I used to carry an extra fan belt, points, condenser, distributor cap and tool kit. These days I don't mess with that stuff...there's nothing mechanical I could possibly fix, either at roadside or at home. I carry my AAA card instead.
I'd like to pretend that I'm more responsible than I am, but sometimes I don't even check to see that I have a spare tire. Since I rarely get flats anymore, I usually do have a spare tire. Although I'm perfectly capable and equipped to change a tire, I can handle that with a cell phone and my AAA card. In the winter, even on shorter trips, I try to remember to place a wool blanket in the car; I have one in plastic for just that purpose. It's a thick, 100% wool blanket, so I think I could survive almost any Maine winter night in a car. Although I will have either my Garmin or my iPhone for GPS, I carry a Maine Atlas for trips within the state when traveling alone because I like to turn off onto roads I haven't been on before. When my wife's along, she doesn't let me get off the Interstate unless it's absolutely necessary. I should carry a small shovel, which I have, and a flashlight, but I usually forget to take it. One might think I could just leave them in the car, but my wife often unloads the trunk; I'm not sure why; maybe she puts bodies in the trunk when she's off by herself, I don't know.
A couple of beach towels, a folding chair, old sneakers, a tarp, a gallon of water and a roll of paper towels, plus the usual jumper cable, spare tire, jack.
Oh yeah. I normally have at least one of those thin rolled up fleece blankets that sell very cheaply in the back of the car. I'm starting to think a small plastic tarp makes sense as well.
We carry a lot of stuff in the car in the winter here, as breaking down in winter here could mean death at -40 F./C. In the summer we carry much less. There are flares and the usual spare tire in the car. In the truck I carry about the same year-round, but we don't take it on long drive. I do have basic tools and fluids, and an emergency sleeping bag plus tarps and ratchet straps and rope.
I still have a "donut spare" but it seems fewer and fewer cars include even those now. A car that I'm sort of looking at only comes with a "fix a flat kit" but there does seem to be a lug at the bottom of the well back there to mount a spare.
I recently bought a car that came with that Fix-a-Flat nonsense. I never thought to look to see if a spare was included. I ended up buying a donut spare kit from an Ebay seller who apparently has found his niche selling spares for all the new vehicles. At least the manufacturers are still putting the tire well in so there's a place for it. If you order a car you can probably include the spare in the build sheet, or just tell the dealer that a spare is a condition of sale. Wish I had done that.
I did find this place: https://modernspare.com/ But it might be a "boutique" solution, it's pricing is a little steep. Of course I was pricing the spare tire & wheel, jack, and tools as a package. Perhaps they aren't out of line even if you buy the "case" option if you don't have a well to put it all into? I haven't asked or checked any regular tire stores. Even the jack comes tailored to your car model so it hits the proper contact point and you won't damage anything. The complete kit even has a tire bag to put the flat tire into so you won't get mud all over after putting on the spare.
Since I'm diabetic & prone to low glucose, I wouldn't last very long without food - especially carbs. I keep several boxes of dried fruit bars. They last several months. Also water. On a long trip, I'll also keep a Glock. If I'm broken down, I'd be vulnerable on a deserted highway.
I looked at those, but this Ebay seller has excellent reviews on the car forum. https://www.ebay.com/itm/326269883737 I don't know what make/model car you're buying but he carries a wide range of donut spares, both with and without the "kit". I bought the kit with the scissor jack and tire iron; those are stored in a felt pouch under the tire.