Check out these online games for exercising your brain:
In Assembler you have to move objects around to solve the puzzle. There are boxes, boards and barrels and they all obey the laws of physics. Each level is a little harder than the last.
FreeRice quizes you on vocabulary, adjusting to your skill level. Might be good practice for preparing for the GRE. There are other quizes too: Math, Foreign languages, Atomic Symbols, Geography and even paintings.
GeoSense tests your knowledge of geography. You have to click on the location of a city within 10 seconds. Your score depends on both speed and accuracy. You can choose US cities, European cities or World cities.