It really depends on the age of the child, but I'll try to deal with the most common and realistic danger.
Especially for adults who aren't use to children - they will put nearly anything in their mouth/nose/ears (some children avoid the nose/ear, but it varies by child). It's their way of exploring their world. But older children are still susceptible, and the CDC gives some nice info on this. Food and candy accounts for about 60%
Beyond the danger of choking, I'm talking about eating/drinking things they just shouldn't - medications (even over the counter 'safe' varieties - pain medicine like Tylenol being all too commonly dangerous), alcohol, tobacco, and things that are for decoration or even pest control! Roach motels, rat poison, fly paper...
When a child is not from a home where these things are present, they might not even know what they are because the parents got rid of things back during baby-proofing and never reintroduced them. This is especially dangerous, because then it's a mystery/fascinating thing and they will naturally want to find out about it. Which leads to...
Again with the poisons - household cleansers, especially strong ones like drain cleaner, bleach, and ammonia are really really not for touching without gloves. Even when gloves aren't needed, children might not realize they need to wash their hands and end up wiping their eyes or touching their mouths...a highly unpleasant discovery at best, and potentially deadly.
When you don't have kids, having some drain cleaner on-hand in-case of a clog is really handy! When kids are going to be around it's best to lock them all up or just not have them - buy and use as needed, and rinse+crush+dispose of the bottle immediately.
Stairs and over-head cabinets can be really dangerous, especially for kids who like to climb and get on top of things! Maybe trying to hang from a door, or get something too high for them because they don't want to bother you - very common sources of injury! The worst is probably the top of the fridge and high cabinets that include heavy things, glass things, and things that are stacked. They just want a plate or a cookie, and then all this stuff they can't see or they just didn't realize how heavy it would be...
Also on the list is unsecured bookshelves (one grab to get a high book or trinket and the whole thing will be on top of them!) and any tall furniture. TV stands, dressers, and bookcases are just so often terrible for vertical stability, and those of us not in an earthquake-zone rarely think to anchor them to a wall! Big tube TVs are still pretty common, and they can even outweigh kids near their teens, while a fully-loaded bookshelf is too much for even an adult to secure themselves against without likely injury if it should topple.
Dangers From Above
While other dangers from above are mentioned, the general rule for kids is they are shorter than adults and see things from a different angle. Especially for kids up to around 8-10, kitchens are especially dangerous. One reach up to a pot to see what's in it (when it's full of boiling water or grease) and there is just no easy way to fix what can result. Putting things on the back burner, communication with kids (lifting them up to show, and having special chairs/stools for them to see up is often helpful, talking to them about what's on the stove, etc), turning pot handles so they aren't available to reach up and touch - even for a long-time parent its easy to forget about and walk away from the stove without thinking, so don't rely on any one method.
Knives are common both in the kitchen and elsewhere in many homes. We all know knives are dangerous, but if you have a nice show-piece hanging on the wall or a utility knife in the closet or a box-cutter under the TV, it's easy to just forget it's even there! But again, this is an extremely common cause of injury...for adults! So it's certainly for kids too.
Similarly, electrical things can be easy to forget about...that frayed wire on the thing that still works but you just don't touch it directly, the extension-cord that barely reaches the TV, that power-outlet that isn't quite right so you avoid using it, the outlet or switch that has no face-plate or cover on it...
Especially when you don't have kids full-time, it's easy to just forget all the things you just "know better" than to do, but that are very easy to do in practice. Like washing your hands and missing that uncovered light switch directly and having your wet fingers slip back in towards the wires.
Drills, saws, knives, nails, screws, hammers (did you ever keep on on TOP of the closet?), chisels, awls...anything in a utility closet, toolkit, garage, etc - it's danger to adults and if you use any you've surely hurt yourself at least a little at some time! For kids it can just be a lot worse. Yet they are extremely normal household goods.
Decorations!? It might sound obvious, but again it's very normal (depending on where you are in the world) to hang inedible berries/plants, swords, antlers/teeth/heads, guns, and knives on the wall. Big paintings with or without glass covering them are in the same general line. As adults, we know they are "for show", not for touching...well, unless we are left alone and it's just sooo cool looking - is that a real samurai sword? I wonder how heavy it is...
But I digress.
Also common are big pots, plants, glass work, small nicknacks (again with the choking and cutting), tall stools that are easily tipped, perhaps heavy furniture or display stands/cabinets. All dangers of various sorts, but when you don't have kids around all the time you just know to avoid certain classes of activities.
Finally, the end our fright-fest: water. What? While vital for life and handy for fun, and even if you turn down your water heater to avoid scalding (good job!), water can be a danger - especially in the form of pools, ponds, tubs, streams, rivers, etc.
Children and guns don't mix well, regardless of education or familiarity with the subject. Lots of experiments show it and often make TV - even parents who think their kids "know better" and have been extensively taught how to handle them properly are shocked how little it takes (especially with multiple kids involved) to prompt them to behave improperly. They're just kids - not trained adult soldiers, who themselves have a terrible habit of not using their guns properly either. Assume kids (and adults!) make mistakes, and just try to avoid making it easy to perform fatal mistakes.
In other words, keep the guns - if you have any - completely, physically inaccessible to kids. Preferably locked and out of site at least while they are around, or whenever not used for a specific purpose. Some adults take kids hunting, for instance, and I personally think that's great - but still, as soon as not using them lock them away to avoid tragedy.
The big "thing that links them all" is novelty/newness makes children want to know more about it and get closer, and no one automatically knows better - and children have had much less time and opportunity to learn. If they can try it, they might.
There is no magic bullet in figuring out these things, but pretending you are "baby-proofing" is a great method, and many great resources can guide you in this process.
So many things are fine for adults "because"...then look closely at the reason. If it's OK for an adult because of experience, training or specific education, or their size/weight/height - it's probably dangerous to a child.
I know this sounds like "all of the things" are dangerous...and, well, it's kind of because they are. But most dangers, once identified, are easily mitigated with a little time and effort - anchoring, proper storage, locked/inaccessible cabinets/rooms, and some good communication and supervision go a long, long way.
Above all - thanks for spending the time an energy to be a responsible adult who looks out for the little ones!