I'm an economics undergrad at Cambridge. My best advice would be to read around and find what you are really interested in. Research it and discuss/critique in your personal statement. Try to give your own perspective on the topic and who/what you agree with the most.
In your personal statement, the most important thing is that you show off some originality and a flexible thought process. Show that you can be critical, that you can research a topic, and that you can offer your own thoughts rather than just borrowing from other people. Of course, if you can weave in details about relevant essays for competitions/ independent research projects/ other roles and activities, then that's great too.
My section on extracurricular was two sentences at the end of my PS where I highlighted the skills I developed. They were never mentioned at any other point in the application process. However, it is still important to have something there.
Also, be sure to practise your interview technique for the final stage of the application process. That's where they make a final decision on whether to admit you, so you should aim to have a good idea on how to approach it. Try to find someone who specialises in economics (a teacher perhaps) to help you out with that.