You can do useful mental exercises for Google interviews, or other interviews, by working through "puzzle questions" and similar problems. But if a world of limited time, this might not be the best use of your prep time.

A good source for the history of these questions and interview style, as well as a bevy of sample questions, can be found in "How Would You Move Mt. Fuji?" http://www.amazon.com/Would-Move-Mount-Microsofts-Puzzle/dp/...