It's cool to toy with Tetris. Did it myself as a student, can't remember all the stuff I tried. But one thing I did was to make the blocks go invisible a few seconds after landing. Fun to try and remember the state of the board.
Another interesting one is that it changes all of the blocks to be monochromatic, including in the "Next Block" preview. It turns out that mode is actually more challenging than you might think; at the speed the blocks are dropping, you depend on peripheral vision to tell you what blocks are upcoming, and now you have to actually examine the block to figure out what it is instead of just knowing from the colour.