I remember writing a naive recursive bruteforce Sudoku solver in TI-BASIC on a TI-82-stats in high school. The "recursive" part was non-trivial, because, though a sub could call itself, there were no local variables, only global variables shared between all the subs. Fortunately, there were global lists that you could use, so I ended up using a global list as the stack, along with a global variable to maintain the nesting depth between the recursive instances and serve as some sort of pointer to the current stack position. It worked; it was so slow it took about one second per call, so you could really see it explore the tree, but it usually managed to solve Sudoku grids in a few hours.

I also recall writing minesweeper (this time on a TI voyage 200), also in TI-basic. The main difficulty, of course, was that when you hit an empty square you have to recursively uncover all adjacent empty squares. So I wrote this recursively, but on this TI there were local variables to recursive calls which also meant that the calculator would sometimes run out of memory when doing its recursive calls (at vaguely unpredictable intervals). Fortunately, I discovered that this could be caught with the rudimentary exception-handling mechanism provided, so I would use this to explore not everything but a small radius around the selected position. The puzzling thing, however, was that the radius was uneven, and got smaller and smaller (as if some memory was being leaked by the interpreter), and more surprisingly, in the graphics view that I used, whenever the exception handling would trigger the TI would display a small warning "differentiating an equation may yield a false equation". To this day I have no idea why.

It all sounds silly now but I learnt a lot typing naive code for a slow, buggy, and limited interpreted, on a cramped calculator keyboard, during boring classes. If smartphones had existed back then, I wonder what it would have changed.