To keep your brain sharp over winter break, the best thing you can do is read. Read widely! If you typically only read mysteries, branch out. Read expository, comics, and newspapers. By doing this, you will stretch your mind, be exposed to new vocabulary, and comprehend new material.
In addition, keep your math skills sharp. Teach someone else what you just learned at school. This makes you the expert and cements your learning.
Finally, be inquisitive! Strive to be a life-long learner. Sometimes schools do not motivate students to achieve this goal. Assigned reading takes away from the intrinsic rewards of reading self-selected material. Let's face it--it can take a lot of time to fulfill the demands of school. Did you know that the amount Anericans read after graduation varies widely? On one end of the spectrum lies people who never read. On the other end of the spectrum lies people who read four books per month. Guess which group contains more millionaires? It's a direct correlation. Which group would you like to join?