Lots of people do the EPQ - it's generally favoured as more worthwhile than Critical Thinking. It can be a lot of work, but it's usually a big project you can talk about in interviews and show you've researched some stuff on your own. I couldn't be bothered with it, personally, as it didn't really have relevance to me doing chemistry.
Also don't bother so much over summer, there's not really much to revise or learn beforehand. That's more important next summer, rather than now. Keep your notes in order when you start doing chemistry as they're often very handy to look back at as more information builds on top of them, and don't panic. Just try and understand, rather than rote learn it, as it will make things quite easy as you go along if you get the fundamentals.