It depends on where your interests lie. Math is all around us and is involved in most everything we do, so any subject will work.
When was the last time something made you stop and think "that's cool--how does it work?" What was it and what about it caught your attention?
Just live your life and something will eventually catch your eye. Every "huh, that's interesting" moment is an opportunity to dissect it and learn the math that's involved.
Find a problem that piques your interest, then learn the math that's relevant to the problem. You can choose a problem from most any subject, like math, physics, or economics.
Having a problem that piques your interest--something to work toward--is a good motivator.
Create a "Taste Genome" comparable to Pandora's "Music Genome" and let people select which foods they like, then recommend foods with similar taste profiles. (foodgenome.com appears to be an abandoned attempt at this.)
Alternately, let people enter the foods they like. Using an algorithm similar to Facebook's suggested friends one, treat the food as friends. If a lot of people who share your taste also like something else, offer it as a suggestion.
This would help picky eaters (e.g., me) venture out and try new foods.
I believe humans will eventually be able to live forever.
Why? I believe our knowledge of biological systems will advance to the point that they can be accurately simulated on a computer within 10-15 years. That will greatly reduce the time it takes to perform experiments (e.g., from months to hours) and take advantage of the knowledge they provide.
At that rate, it's highly probable that a method to prevent--and reverse--biological aging will be discovered.
Figure out how you want to change the world. Use OCW, if necessary, to get a general idea of how to do it. (If it's a scientific issue, for example.)
Get to know researchers and PhD students at local universities. Offer individual grants to those with the expertise you need to help you change the world. (Again, science-oriented solutions.)
When was the last time something made you stop and think "that's cool--how does it work?" What was it and what about it caught your attention?
Just live your life and something will eventually catch your eye. Every "huh, that's interesting" moment is an opportunity to dissect it and learn the math that's involved.