There a a few definitions of math whiz that are commonly used in society.  One is the ability to do numerical calculations in his/her head quickly and accurately, and the other is strong problem solving combined with abstraction skills.  The first is an excellent ability to have in everyday life and allows that person to estimate numerical values at anytime with little effort.

The second skill however, is much more valuable, as that person is unlikely to be fooled by media logic holes (of which there are many), fallacies, and the ability to solve problems.  Many employers will pay handsomely for a good problem solver, since there many people are not effective problem solvers.  Many of the world's top CEOs are in fact former engineers or mathematicians, and are extremely wealthy from their mathematical skills.

There are plenty of ways to use math for fun just as there are plenty of ways to use literature for fun.  A person who enjoys literature and English class might complete crossword puzzles in their spare time.  A person who is more interested in mathematics might complete sudoku puzzles instead.  There are many math games and puzzles just like literary and vocabulary games and puzzles.

This is a more simple suggestion. Just push yourself to learn more. Whatever math you're taking now, see if you can get a textbook for the next level and try to make sense of it. Or challenge yourself to do increasingly difficult problems. It has always seemed to me that math really is a habit of mind, and that if you don't use it and hone it, those habits can weaken over time. So practice, and push yourself.

So you're talking about doing stuff for fun with your math abilities, right?  You could design things using math.  For example, you could try to design and build a bridge using toothpicks and determine what weight it would hold.  Then you could test it out by putting weight on until it cracks.

If you are asking for activities you, as a ninth grader can do, I would suggest you consider finding a group of senior citizens who want help setting up computer systems to track their investments, pay bills, and so forth. You will need to convince them that you are absolutely honest and responsible and will respect the privacy of any information they provide, but there are lots of good programs that seniors could use once they were shown (and probably shown again and again) how to use it.

This isn't directly using your gifts with mental math, but it relates and would be much appreciated by those you helped!

Like you can do writing for your own pleasure, you can solve mathematical riddles or investigate some mathematical subject for the pleasure of doing it. 

The more you challenge your brain, the better thinker you become. Working with mathematics, improve your logical thinking. 

As for something to do, what about tutoring. Volunteer at a community center or at school, and help other kids. This is will be an excellent addition to you CV.