I'm in the same position as you, what I've done and plan to do is:
1) Buy the science and maths textbooks, and read through them, taking very general notes on each topic just so that I understand the general idea. Not any memorisation, just to make life easier when the workload gets bigger and I need to 'get' things instantly.
2) Find a reliable GCSE vocab list for AQA French (or whatever board/language you're doing) and put some of the general vocab into Quizlet. Nothing topic-specific, just things like connectives and time phrases, to amp up my writing/speaking skills.
3) Reorganise myself and my organisation system to make it quicker and easier to work during GCSE.
4) Reading French news every morning and looking up unfamiliar phrases. On a similar note, listening to French music and watching French movies.
5) Going through what I've done during year nine and condensing the year's notes into one key file, to make revision easier later on.
6) Looking up TSR's GCSE resources for all the subjects I'm taking, and favouriting ones that look the most useful from each key topic. Then, if they come up in school later on, I have a quick way to find more information.
7) Watching revision videos on YouTube and researching creative revision tips, and noting them down to use later during exam season so I don't get stuck just using flashcards again.
I'm doing quite a big amount and it's totally not needed, but I'd say just pick and choose whatever sounds appealing!