My advice would just be to read what interests you. I know this really doesn't seem helpful, but it's best for you to find an area of the course that stands out to you and go from there. The books which you think you should read are probably what everyone has read, so don't just stick to Freakonomics. I read quite a bit about globalisation for example, but you could really choose any topic, just be wary that you will be expected to actually know some stuff about it in an interview. Be careful in your PS that you're not just name dropping books or economists, I'd take just one or two and talk about them in more detail. Also, remember that Oxford (or Cambridge if you change your mind
) is just one of your choices and although they don't care so much about extra-curriculars, other unis will so it's essential to include a little bit at the end about these.
Activity wise, there were a couple of essay competitions for A-level students when I applied, assuming they're still around you could look at them. That being said, I entered and didn't get placed or anything, ended up not mentioning it and it didn't harm my application. You could also look to see if there are any lectures or talks near you (probably easier if you're in/around London) or there are so many online courses you could work your way through. Essentially, you just need to show that you're interested in your subject, and also that you actually know something about it.
Number one interview tip: prep for the TSA or you won't get an interview. Make sure you practice sitting a whole test in timed conditions as the timing is the worst bit. Interviews vary a bit by college but I personally had two interviews - one for econ and one for management. The econ one was quite mathsy, though the maths itself was relatively simple and it was more a case of figuring out how to apply it to the situation. Management was more common sense and problem solving, you didn't really need to have any prior knowledge, we just worked through a problem together.
Basically for the interviews, I would say that it's most important to try to remain calm (easier said than done, I know), think out loud so that they can see how you're tackling the questions they give you, and try to show an interest in what they're saying. I firmly believe that my lack of knowledge surrounding some of the things they asked in the interview was made up for by the fact I tried to seem interested and enthusiastic about the topics