I am also in Year 12 and I am trying to use this year to its full potential. Currently, I am taking four A-Levels in the forms of chemistry, biology, maths and physics; however I am strongly considering dropping physics, seeing as three A-Levels are probably stressful enough and I would rather get, for example, A*A*A* than A*AAA or A*A*AA.
With regards to extra curricular activities:
I have a part-time job at a supermarket; part of a team in the Dragons' Apprentice challenge (similar to Young Enterprise); Silver DofE (almost finished); vice-house captain for one of the four houses in my school; I attend the gym and play football and Extended project qualification.