In the right hands, a calculator can be not a force for mathematics but a source of endless fun. I'm not just talking about writing rude words upside-down, either; how about loading Super Mario on the TI-84.
You'll need the Super Mario v1.2 program source from here, which comes with Bill Nagel graphics, three unique enemies, 64 unique background tiles (some with animation), powerups, fireballs and an end-of-level boss. There's also a World Editor app that will run on the calculator itself.
There's a video demo of not only the game itself but the installation procedure here. This could've kept me far more occupied in math lessons back at school.