I built a checking program in C# to mainly use as a check register and balancing tool. Doesn't do double entry accounting. Also build a simple notes program in both C# and python that can use the same data file. Also did a cheap and dirty internet radio player on an old Rasp. pi. Used one push button to cycle thru a list of internet radio feeds. Plus an led that flashed the index of the list which gave me an idea of which station I was on.