Well, I can't tell you low-hanging fruit that's still out there, because if I knew it, I'd be working on it instead of telling you. :) But I can tell you some low-hanging fruit that I've done!
I'll stick to just one example, this[0] paper of mine was, like... I was astonished this was not already in the literature -- everything in this paper is easy to prove once you think to ask the question -- but as best as I and anyone else could tell, somehow nobody had thought to ask the question despite having asked other extremely similar ones. It's especially astonishing that Jacobsthal's exponentiation was independently rediscovered multiple times, but what I call "super-Jacobsthal exponentiation" seems to have only been considered once before and only briefly as a tool for something else (nobody thought to write down its algebraic laws). So, I wrote it up to make sure it was out there.
(Honestly I'd say a lot of my work has been LHF, I got through grad school while learning hardly any of the heavy machinery one normally does, unfortunately most of it isn't written up yet and so I can't really easily point to it.)
There's plenty examples indeed if you just dig around. I have a colleague who published a theoretical paper [0] in one of the top fluid mechanics journals 5 years back. That paper originated from his lecture notes in a course that he gave for the first time, where in one part he followed the classical theory of waves behind a ship done by Lord Kelvin and others back in the olden days. Then he thought "Hmm, how do we extend this so it works if there is a current in the water?" And it turned out to be both possible to solve analytically, and that nobody had done it before. Nobody had thought it was possible to do. It's an example of "low-hanging fruit" that had been sitting since 1887.
I think the reason why there's "lots" of LHF is that science has become so incremental, iterating on recent results. If you go back 20-50-100 years and look at the road less traveled, there's plenty of LHF, but going down that path and shaking the trees requires more effort per initial publication than most academics can afford under the current system. But if you can afford (or get lucky enough) to find that first LHF, it usually gives you enough material to work on for quite some time such that it pays off over time.
I'll stick to just one example, this[0] paper of mine was, like... I was astonished this was not already in the literature -- everything in this paper is easy to prove once you think to ask the question -- but as best as I and anyone else could tell, somehow nobody had thought to ask the question despite having asked other extremely similar ones. It's especially astonishing that Jacobsthal's exponentiation was independently rediscovered multiple times, but what I call "super-Jacobsthal exponentiation" seems to have only been considered once before and only briefly as a tool for something else (nobody thought to write down its algebraic laws). So, I wrote it up to make sure it was out there.
(Honestly I'd say a lot of my work has been LHF, I got through grad school while learning hardly any of the heavy machinery one normally does, unfortunately most of it isn't written up yet and so I can't really easily point to it.)
[0]https://arxiv.org/abs/1501.05747 (This, um, will require some knowledge of ordinals to read.)