A good quality game is snake. Can't go wrong there. Another hit that usually is right on the calculator is "block dude". I've wasted many a day playing that one.
There are games of various genres available to be played in graphing calculator. These games can range from simple old arcade classics like pong and astereoids to puzzle games like sudoko to RPGs like bandcamp and bleach to even first person shooter games like clones of the popular id software games doom and quake.
If you have an hp graphic calculator, you have tetris o it. Just type MINEISBETTER in the equation writer environment, hit EVAL and voila. Or you can draw something using the graphics capabilities.