Citizen science is a worthwhile and effective hobby.  In your spare time you can be an active part of ongoing research projects by classifying moon craters, identifying animals in the wild, and model the Earth's climate with historic ship logs.  

Khan Academy
Using innovative learning tools, Khan Academy is free and available for anyone who wants a second shot at trigonometry.  Or chemistry, economics, computer programming etc.  Your only obstacle is the speed of your internet connection.

Code Academy
Like Khan Academy, Code Academy is free and open to anyone with the desire to learn computer programming.