Wow such broad questions.
I'm not a Cambridge student but I'm an undergraduate applicant and I'll answer from the knowledge I've accumulated:
Work experience - no, not important unless you can show how it has helped you gain exposure into the academic study of the subject. For land economy, this is near impossible.
Reading useful books - very useful and important. I believe land economy is comprised of three different subject areas (right?) so reading around those areas, especially the parts which you find interesting, will undoubtedly help you write a personal statement and perform well at interview.
Extra-curricular - Cambridge states that all admissions decisions are made on entirely academic merit. However, I'm personally convinced that when it comes to finely differentiating candidates, a nice profile of extra-curricular activities to show some 'personality' may be beneficial. But the official line is that they don't matter for the sake of admission, but you'd be a pretty boring person if all you do is study!
Involvement in school - well it will help as your school write your reference. I was (and still am) massively involved but my school recognised none of it in the reference, which was probably because my crappy tutor rushed it in 10 minutes and the senior staff nodded it through without any concern for checking (bar one single helpful teacher). Again, if it isn't academic then Cambridge officially don't rate it when considering whether or not to make an offer but it can't harm.
As I think you've gauged, grades and marks are important so make sure you pick the right subjects and work hard. You can't work so hard that you'll achieve more than you're capable of, but don't self yourself short by underachieving when you could score higher.
(P.S. I take no responsibility if any of this information is wrong, inaccurate or misleading. I am not and do not claim to be a Cambridge student, so please let some of them corroborate what I've said before taking it as gospel).