Open Arena is a good shooting game, you can get it from the Software center in ubuntu.
Holy spirit is another game similar to Diablo but for linux here are the instructions http://www.ubuntuvibes.com/2012/06/h...ce-diablo.html
TORC is a car racing game available in the ubuntu Software Center too
Super Tux 2 is a Super Mario game like with different worlds to go, also from the software center
Extreme Tux Racer is also another game available fromt the software center clean entertaining
DESURA is a STEAM like web site with lots of games that works in Linux http://www.desura.com/
Angry Birds and some more games are available to play right from the Google Chrome web browser too
You can check all these games out in youtube before installing if you like any of them there is always people there with tips for games and stuff.
Good luck to you.