During high school the only computer I had access to during school days was my beloved TI-84+ graphing calculator. It was a surprisingly enjoyable platform for practicing programming. The built-in TI-BASIC programming editor and simple drawing commands meant that it was easy to create small graphical demos and games, which is what I did obsessively any chance I got - at lunch, during class, in the hallway, on the bus, etc. I think it's likely I spent more than a thousand hours writing code on the little gadgets. Enough that I wore through the keypads on a few of them.
Below are the results of some of that time: