For German, I would definetly recommend reading, watching films / episodes (English subtitles are fine) and learning more about the culture. I personally found that my German GCSE covered very little culture, and there will be a lot more about it at A Level, which is really interesting!
How do you find German grammar? If your school has put a fair amount of emphais on grammar then you're already ahead of most people in Y11! A few people in my German A Level class had done very little grammar before, and that was the biggest transition for them. Don't stress out about it if you can't / don't want to do this, I totally understand that grammar can be less interesting than other areas of languages. It might be worth familiarising yourself with the declension tables, cases and propositions. But again don't worry about it, as your teachers will cover this at the beginning of Y12, this is just a way to get ahead. If it comes to choosing between reading / watching / culture and grammar, do the former, not the latter. It's more fun and none of this is compulsary! Have fun and Happy Easter!