I don’t do a lot of gaming, but I have enjoyed playing Portal. It is game where you run around with a gun that can shoot a blue or orange doorway. You can walk through one doorway and come out the other. The game revolves around solving puzzles by using these doorways to throw yourself over wall, drop objects on attacking robots and redirect energy balls.
The game runs on Steam which until recently was a windows only game distribution system by the makers of the Half-Life series of games. Until yesterday I had to use an application called Cross Over Games to play on my Mac. (Cross Over Games basically emulates Windows so the app will run. However, they just came out with a Mac version of Steam and are working on porting other games as well.
They are making Portal freely available for awhile, so if you’d like to try it out now is a great time. They also have a number of other games available that will work on both Mac and Windows. Another puzzle game I’d recommend is World of Goo. My daughter and I are working our way through it and it has been a lot of fun–its nice to have a decent game you can play with a three year old helping.