You see, one cannot become an expert in Physics, without tackling problems. Especially, in Theoretical Physics, math plays a dominant role. One of my favorite hobbies is digging Physics problems. One of my favorite books in this regard is the Princeton Problems in Physics. Almost all aspects of Modern Physics are covered in this great book and working out the problems is a great stress breaker. The best part about this book is that you have the entire solutions available as well.
If you have a passion for Physics and wanna train yourself, this problems book is a great addition. One of my favorite pass times lately has been developing programs to solve many problems presented in the book. Totally awesome feeling!