Yeah, tutoring is a great way to consolidate what you know and proves that you understand maths well enough to teach it (which is a big plus!). It builds up your CV and maybe even your personal statement. Also, you could try an online maths course - there are loads of free ones on Coursera etc... and they're always useful I find. Books are always good for extra stuff before uni: Fermat's Last Theorem by Simon Singh is apparently really interesting.
Hope any of that's useful!...