Computer Science Club Activity Ideas

I'm in year 11 and I take Gcse computer science. My teacher has asked me to help out at a new club which is aimed to get more girls interested in this subject! The club is for girls in year 7-9. Does anyone have any ideas of computer science activities that could be fun for this? It would need to be something challenging yet achievable for younger students. And maybe something everyone could get involved in together? Not sure exactly of the sorts of things. Any ideas appreciated!!