Try different things. Learn a new language. Some times in the process of learning that other language, actually makes you better on both.
Contribute to an open-source project.
High scalability and PHP/MySQL are not friends . There are technologies today that are build just for that. node.js and Redis to name a couple.
Answer questions in SO. Other people's problems can make you think outside the box and you improve on the process of trying to help, and you also do something nice to someone.
[ will try to add here if I think of anything else I just woke up drinking coffee :D ]
